drivers/memstick/core/memstick.c: avoid -Wnonnull warning

gcc-7 produces a harmless false-postive warning about a possible NULL
pointer access:

drivers/memstick/core/memstick.c: In function 'h_memstick_read_dev_id':
drivers/memstick/core/memstick.c:309:3: error: argument 2 null where non-null expected [-Werror=nonnull]
memcpy(mrq->data, buf, mrq->data_len);

This can't happen because the caller sets the command to 'MS_TPC_READ_REG',
which causes the data direction to be 'READ' and the NULL pointer not
accessed.

As a simple workaround for the warning, we can pass a pointer to the
data that we actually want to read into. This is not needed here, but
also harmless, and lets the compiler know that the access is ok.

Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Arnd Bergmann <[e-mail]>
Cc: Alex Dubov <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/memstick/core/memstick.c

···
 	struct ms_id_register id_reg;
 
 	if (!(*mrq)) {
-		memstick_init_req(&card->current_mrq, MS_TPC_READ_REG, NULL,
+		memstick_init_req(&card->current_mrq, MS_TPC_READ_REG, &id_reg,
 				  sizeof(struct ms_id_register));
 		*mrq = &card->current_mrq;
 		return 0;