Input: evdev - try to wake up readers only if we have full packet

We should only wake waiters on the event device when we actually post
an EV_SYN/SYN_REPORT to the queue. Otherwise we end up making waiting
threads runnable only to go right back to sleep because the device
still isn't readable.

Reported-by: Jeffrey Brown <jeffbrown@android.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>