USB: ssu100: add disconnect function for ssu100

Add a disconnect function to the functions of this device.  The
disconnect is a call to usb_serial_generic_disconnect() so it requires
that symbol to be exported from generic.c.

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>