usb-serial: use new registration API in staging drivers

This patch (as1530) modifies the following usb-serial drivers from
drivers/staging to utilize the new usb_serial_{de}register_drivers()
routines:

	quatech_usb2 and serqt_usb2.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>