getevent: Fix a couple of bugs.

Fixed a bug printing the event value when using labels.

Stop trying to print the available codes for EV_SYN because
we cannot actually query them.  EVIOCGBIT(0, size) is a special
case that returns the set of events that are supported, and
EV_SYN == 0.

Change-Id: Iea086ba24300ca0815e4814a3bc5ff60756612c2