Input: ir-keytable - fix uninitialized variable warning

We were forgetting to set up proper return value in success path causing
ir_getkeycode() to fail intermittently:

drivers/media/IR/ir-keytable.c: In function 'ir_getkeycode':
drivers/media/IR/ir-keytable.c:363: warning: 'retval' may be used
uninitialized in this function

Reported-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>