memstick: remove mspro_block_mutex

mspro_block_mutex is identical in scope to mspro_block_disk_lock and therefore
unnecessary.

Signed-off-by: Alex Dubov <oakad@yahoo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>