USB: serial: mos7720: fix error code in mos7720_write()

This code should return -ENOMEM if the kmalloc() fails but instead
it returns success.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 0f64478c ("USB: add USB serial mos7720 driver")
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>