Remove magic numbers in m68k-dis.c:print_insn_arg

Remove magic numbers in m68k-dis.c:print_insn_arg

When I inspect the return values of disassmblers, I happen to see
various -1/-2/-3 magic numbers are used in m68k-dis.c.  This patch
is to replace them with enum.

-1 and -2 is "clearly documented" in print_ins_arg's comments, but
-3 isn't.  In fact, -3 is returned when FETCH_DATA returns false,
which means memory error (because fetch_data return 0 on memory
error).  So I name enum PRINT_INSN_ARG_MEMORY_ERROR for -3.

This patch is a refactor patch, doesn't affect any functionality.

opcodes:

2017-01-13  Yao Qi  <yao.qi@linaro.org>

	* m68k-dis.c (enum print_insn_arg_error): New.
	(NEXTBYTE): Replace -3 with
	PRINT_INSN_ARG_MEMORY_ERROR.
	(NEXTULONG): Likewise.
	(NEXTSINGLE): Likewise.
	(NEXTDOUBLE): Likewise.
	(NEXTDOUBLE): Likewise.
	(NEXTPACKED): Likewise.
	(FETCH_ARG): Likewise.
	(FETCH_DATA): Update comments.
	(print_insn_arg): Update comments. Replace magic numbers with
	enum.
	(match_insn_m68k): Likewise.