Staging: line6: replacing -EINVAL with retval

drivers/staging/line6/midi.c:147 send_midi_async() 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>