spu: make some constants unsigned

spu: make some constants unsigned

The field in spu_opcode is unsigned, and for some values of opcode we can end
up shifting into the high bit.  So avoid possibly creating a negative number
and then assigning it to a unsigned field by shifting an unsigned constant.

gas/ChangeLog:

2016-05-23  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-spu.c (APUOP): Use OPCODE as an unsigned constant.