drivers/media/IR/ir-keytable.c: fix binary search

The input-large-scancode patches changed the binary search in
drivers/media/IR/ir-keytable.c to use unsigned integers, but
signed integers are actually necessary for the algorithm to work.

Signed-off-by: David Härdeman <david@hardeman.nu>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>