USB: serial: mos7840: add support for MCS7810 devices

This patch added the support of MCS7810 device for the mos7840 driver.
The MCS7810 device supports single USB2.0-to-Serial port with
a LED indicator for reflecting transmission or reception activity.

Signed-off-by: Donald Lee <donald@asix.com.tw>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>