W1: w1_family, remove unused variable need_exit

Removed the w1_family structure member variable need_exit.  It was only
being set and never used.  Even if it were to be used it is a polling type
operation.

Signed-off-by: David Fries <david@fries.net>
Signed-off-by: Evgeniy Polyakov <johnpol@2ka.mipt.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>