USB: qcserial: make AT URCs work for Sierra Wireless devices

Three Sierra Wireless modems have been found to require the CDC ACM
SET_CONTROL_LINE_STATE request on the AT port in order to receive
unsolicited response codes, most recently the Dell Wireless 5808e
(which is a re-branded Sierra Wireless EM7355). On the other hand,
the Sierra Wireless MC7710 does not seem to need this request, but
it was found to work either way.

Use this request on the AT port of devices with the Sierra Wireless
layout in the qcserial driver. The other modems that were moved to
the option driver to work around this can now be moved back.

Cc: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David Ward <david.ward@ll.mit.edu>
Signed-off-by: Johan Hovold <johan@kernel.org>