[PATCH] IB/mthca: release mutex on doorbell alloc error path

Release mutex on error return path from mthca_alloc_db().

Signed-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>