[media] s5p-tv: fix wait_event_timeout return handling

event API conformance testing with coccinelle spatches are being
used to locate API usage inconsistencies this triggert with:
./drivers/media/platform/s5p-tv/mixer_reg.c:364
        incorrect check for negative return

Return type of wait_event_timeout is signed long not int and the
return type is >=0 always thus the negative check is unnecessary.
An appropriately named variable of type long is inserted and the
call fixed up aswell as the negative return check dropped.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>