USB: qcserial: update comment for Sierra Wireless MC7304/MC7354

This comment is ambiguous since there are other MC73xx devices with
different USB IDs. This USB ID is found in the MC7304 and MC7354.

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