sh{,64}: make arg type enum

sh{,64}: make arg type enum

The values are always members of the enum, except the two places -1 is assigned
only to playcate -Wuninitialized because gcc isn't or at least didn't used to
be smart enough to figure out its only used if it was set.

gas/ChangeLog:

2016-06-05  Trevor Saunders  <tbsaunde+binutils@tbsaunde.org>

	* config/tc-sh.c (parse_reg): Change type of mode argument to
	sh_arg_type.
	(get_operand): Adjust.
	(insert): Change type of how to bfd_reloc_code_real_type.
	(insert4): Likewise.
	* config/tc-sh64.c (shmedia_get_operand): Adjust.
	(shmedia_parse_reg): Change type of mode to shmedia_arg_type.