USB: serial: metro-usb: convert to use module_usb_serial_driver

Now that we aren't doing anything special in the init function, move to
use the easier module_usb_serial_driver() call instead, saving a lot of
lines of unnecessary code.

Cc: Aleksey Babahin <tamerlan311@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>