w1: fix w1_remove_master_device() searching

In case bus master driver provided bogus value as its private data, search
can be incorrect.  Problem found by Adrian Bunk.

Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Cc: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>