Return 'int' rather than 'unsigned short' in avrdis_opcode

Return 'int' rather than 'unsigned short' in avrdis_opcode

avrdis_opcode return type is unsigned short, but -1 at the end of
this function is returned.  Additionally, print_insn_avr doesn't
handle when -1 (in case of memory error) is returned from
avrdis_opcode.

This patch changes avrdis_opcode returning int indicating the error,
and adds a new argument for instruction we got on success.  The
opcode is 16-bit, so I change local variables type to uint16_t,
and include "bfd_stdint.h" as a result.  On memory error,
print_insn_avr returns -1, which is a common convention among most
of print_insn_$ARCH functions.

opcodes:

2016-12-29  Yao Qi  <yao.qi@linaro.org>

	* avr-dis.c: Include "bfd_stdint.h"
	(avrdis_opcode): Change return type to int, add argument
	insn.  Set *INSN on success.
	(print_insn_avr): Check return value of avrdis_opcode, and
	return -1 on error.