usb-serial: use new registration API in [d-h]* drivers

This patch (as1524) modifies the following usb-serial drivers to
utilize the new usb_serial_{de}register_drivers() routines:

	digi_acceleport, empeg, ftdi_sio, funsoft, garmin_gps,
	and hp4x.

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