include/linux/byteorder: Always defines __fswab64, __swab64p and __swab64s

When __BYTEORDER_HAS_U64__ is not defined, we got warning following:

-----
/tmp/include/linux/byteorder/little_endian.h: In function ‘__cpu_to_be64p’:
/tmp/include/linux/byteorder/little_endian.h:71:2: warning: implicit declaration of function ‘__swab64p’
		[-Wimplicit-function-declaration]
-----

Usually, __arch__swab64* required for __fswab64,  __swab64p and __swab64s
is defined.  Therefore, __BYTEORDER_HAS_U64__ is unnecessary.
This removes __BYTEORDER_HAS_U64__.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
CC: Kim Phillips <kim.phillips@freescale.com>
Reviewed-by: Kim Phillips <kim.phillips@freescale.com>