usb: gadget: acm: fix endianness in notifications

The gadget code exports the bitfield for serial status changes
over the wire in its internal endianness. The fix is to convert
to little endian before sending it over the wire.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Tested-by: 家瑋 <momo1208@gmail.com>
CC: <stable@vger.kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>