mailbox: pcc: Handle all PCC subtypes correctly in pcc_mbox_irq

Commit c45ded7e1135 ("mailbox: pcc: Add support for PCCT extended PCC
subspaces(type 3/4)") enabled the type3/4 of PCCT, but the change in
pcc_mbox_irq breaks the other PCC subtypes.

The kernel reports a warning on an Ampere eMag server

-->8
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.16.0-rc4 #127
Hardware name: MiTAC RAPTOR EV-883832-X3-0001/RAPTOR, BIOS 0.14 02/22/2019
Call trace:
dump_backtrace+0x0/0x200
show_stack+0x20/0x30
dump_stack_lvl+0x68/0x84
dump_stack+0x18/0x34
__report_bad_irq+0x54/0x17c
note_interrupt+0x330/0x428
handle_irq_event_percpu+0x90/0x98
handle_irq_event+0x4c/0x148
handle_fasteoi_irq+0xc4/0x188
generic_handle_domain_irq+0x44/0x68
gic_handle_irq+0x84/0x2ec
call_on_irq_stack+0x28/0x34
do_interrupt_handler+0x88/0x90
el1_interrupt+0x48/0xb0
el1h_64_irq_handler+0x18/0x28
el1h_64_irq+0x7c/0x80

Fixes: c45ded7e1135 ("mailbox: pcc: Add support for PCCT extended PCC subspaces(type 3/4)")
Reported-by: Justin He <[e-mail]>
Tested-by: Justin He <[e-mail]>
Signed-off-by: Sudeep Holla <[e-mail]>
Signed-off-by: Jassi Brar <[e-mail]>

drivers/mailbox/pcc.c

···
 	if (ret)
 		return IRQ_NONE;
 
-	val &= pchan->cmd_complete.status_mask;
-	if (!val)
-		return IRQ_NONE;
+	if (val) { /* Ensure GAS exists and value is non-zero */
+		val &= pchan->cmd_complete.status_mask;
+		if (!val)
+			return IRQ_NONE;
+	}
 
 	ret = pcc_chan_reg_read(&pchan->error, &val);
 	if (ret)