w1: decrement slave counter only in ->release() callback

Decrement the slave counter only in ->release() callback instead of both
in ->release() and w1 control.

Patch is based on debug work and preliminary patch made by Henri Laakso.
Henri noticed in debug that this counter becomes negative after w1 slave
device is physically removed.

Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Cc: Henri Laakso <henri.laakso@wapice.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>