Staging: line6: replacing -EINVAL with retval in driver.c

drivers/staging/line6/driver.c:208 line6_send_raw_message_async_part()
info: why not propagate 'retval' from usb_submit_urb() instead of (-22)?

Signed-off-by: Ashvini Varatharaj <ashvinivaratharaj@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>