usb-serial: use new registration API in [k-m]* drivers

This patch (as1526) modifies the following usb-serial drivers to
utilize the new usb_serial_{de}register_drivers() routines:

	keyspan, kl5kusb105, kobil_sct, mct_u232, mos7720,
	mos7840, and moto_modem.

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