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>
diff --git a/include/usb.h b/include/usb.h
index b8f2fa2..d20efb1 100644
--- a/include/usb.h
+++ b/include/usb.h
@@ -265,13 +265,13 @@
 		((x_ & 0xFF000000UL) >> 24)); \
 	})
 
-#ifdef LITTLEENDIAN
+#ifdef __LITTLE_ENDIAN
 # define swap_16(x) (x)
 # define swap_32(x) (x)
 #else
 # define swap_16(x) __swap_16(x)
 # define swap_32(x) __swap_32(x)
-#endif /* LITTLEENDIAN */
+#endif
 
 /*
  * Calling this entity a "pipe" is glorifying it. A USB pipe