Input: omap-keypad - add missing input_sync()

Otherwise the updated evdev driver (commit cdda911c,
"Input: evdev - only signal polls on full packets") no longer works on
top of omap-keypad.

Tested on Amstrad Delta.

Signed-off-by: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>