USB: serial: mos7840: clean up device-type handling

The current device-type detection is fragile and can't really be relied
upon. Instead of sprinkling device-id conditionals throughout the
driver, let's use the device-id table to encode the number of ports and
whether the device has a driver-controlled activity LED (MCS7810).

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>