usb.h: use standard __LITTLE_ENDIAN from Linux headers

Rather than forcing people to define a custom "LITTLEENDIAN", just use the
__LITTLE_ENDIAN one from the Linux byteorder headers that every arch is
already setting up.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Remy Bohmer <linux@bohmer.net>