USB: mos7840.c: turn this into a serial driver

The MOS driver is "interesting", in a bad kind of 'how the hell did this
get merged' kind of way

- Remove the bogus termios change check
- Remove the duplicate code for half the ioctls
- Remove the supporting code to duplicate the ioctl code

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>