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/20170111144143.548867-1-arnd@arndb.de
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>