usb-serial: use new registration API in [n-p]* drivers

This patch (as1527) modifies the following usb-serial drivers to
utilize the new usb_serial_{de}register_drivers() routines:

	navman, omninet, opticon, option, oti6858, and pl2303.

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