input: evdev: only allow reading events if a full packet is present

Without this, it was possible for the reader to get ahead of packet_head.
If the the input device generated a partial packet *right* after the
reader got ahead, then we can get into a situation where the device is
marked readable but read always returns 0 until the next packet is
finished (i.e a SYN is generated by the input driver).

This situation can also happen if we overflow the buffer while a reader
is trying to read an event out.

Change-Id: If01ab371bc7de1bf1f90c122dcc5a29242b01a09
Signed-off-by: Dima Zavin <dima@android.com>