coresight: Fix erroneous memset in tmc_read_unprepare_etr

At the end of a trace collection, we try to clear the entire buffer
and enable the ETR back if it was already enabled. But, we would have
adjusted the drvdata->buf to point to the beginning of the trace data
in the trace buffer @drvdata->vaddr. So, the following code which
clears the buffer is dangerous and can cause crashes, like below :

memset(drvdata->buf, 0, drvdata->size);

Unable to handle kernel paging request at virtual address ffffff800a145000
pgd = ffffffc974726000
*pgd=00000009f3e91003, *pud=00000009f3e91003, *pmd=0000000000000000
PREEMPT SMP
Modules linked in:
CPU: 4 PID: 1692 Comm: dd Not tainted 4.7.0-rc2+ #1721
Hardware name: ARM Juno development board (r0) (DT)
task: ffffffc9734a0080 ti: ffffffc974460000 task.ti: ffffffc974460000
PC is at __memset+0x1ac/0x200
LR is at tmc_read_unprepare_etr+0x144/0x1bc
pc : [<ffffff80083a05ac>] lr : [<ffffff800859c984>] pstate: 200001c5
...
[<ffffff80083a05ac>] __memset+0x1ac/0x200
[<ffffff800859b2e4>] tmc_release+0x90/0x94
[<ffffff8008202f58>] __fput+0xa8/0x1ec
[<ffffff80082030f4>] ____fput+0xc/0x14
[<ffffff80080c3ef8>] task_work_run+0xb0/0xe4
[<ffffff8008088bf4>] do_notify_resume+0x64/0x6c
[<ffffff8008084d5c>] work_pending+0x10/0x14
Code: 91010108 54ffff4a 8b040108 cb050042 (d50b7428)

Since we clear the buffer anyway in the following call to
tmc_etr_enable_hw(), remove the erroneous memset().

Fixes: commit de5461970b3e9e1 ("coresight: tmc: allocating memory when needed")
Cc: Mathieu Poirier <[e-mail]>
Signed-off-by: Suzuki K Poulose <[e-mail]>
Signed-off-by: Mathieu Poirier <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/hwtracing/coresight/coresight-tmc-etr.c

···
 	if (local_read(&drvdata->mode) == CS_MODE_SYSFS) {
 		/*
 		 * The trace run will continue with the same allocated trace
-		 * buffer. As such zero-out the buffer so that we don't end
-		 * up with stale data.
-		 *
-		 * Since the tracer is still enabled drvdata::buf
-		 * can't be NULL.
+		 * buffer. The trace buffer is cleared in tmc_etr_enable_hw(),
+		 * so we don't have to explicitly clear it. Also, since the
+		 * tracer is still enabled drvdata::buf can't be NULL.
 		 */
-		memset(drvdata->buf, 0, drvdata->size);
 		tmc_etr_enable_hw(drvdata);
 	} else {
 		/*