USB: serial: mos7840: fix error code in mos7840_write()

This should return -ENOMEM instead of 0 if the kmalloc() fails.

Fixes: 3f542974 ("USB: Moschip 7840 USB-Serial Driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <johan@kernel.org>