avr: replace sentinal with iteration from 0 to ARRAY_SIZE

avr: replace sentinal with iteration from 0 to ARRAY_SIZE

This seems a little easier to understand than using a sentinal, and will
hopefully let the compiler optimize the loop better.  It also has the effect
that we stop initializing a field of the sentinal that is an enum with zero.

gas/ChangeLog:

2016-06-01  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-avr.c (avr_parse_cons_expression): Replace iteration to
	sentinal with iteration to array size.