Use register_size () instead of MAX_REGISTER_SIZE

2017-01-18  Alan Hayward  <alan.hayward@arm.com>

	* amd64-tdep.c (amd64_pseudo_register_read_value): remove
	MAX_REGISTER_SIZE.
	(amd64_pseudo_register_read_value): Likewise.
	* remote.c (fetch_register_using_p): Remove MAX_REGISTER_SIZE.
	(store_register_using_P): Likewise.
	* regcache.c (regcache_xfer_part): Likewise.