[ARM] 3755/1: dmabounce: fix return value for find_safe_buffer

Patch from Kevin Hilman

Previous locking changes to dmabounce incorrectly return non-NULL even
when buffer not found.  Fix it up.

Signed-off-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>