drivers/w1/masters/omap_hdq.c: fix missing mutex unlock

This was found using a semantic patch, more info can be found at:
http://www.emn.fr/x-info/coccinelle/



Signed-off-by: Stoyan Gaydarov <sgayda2@uiuc.edu>
Acked-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>