USB: serial: mos7840: rip out broken interrupt handling

The interrupt handling is completely broken and has in fact never been
been enabled due to an inverted test for an interrupt endpoint in
open() that prevented the interrupt URB from being submitted.

Other highlights include missing interrupt URB resubmission (had it
ever been submitted), missing locking when managing the open-port count,
and NULL-pointer dereferences that could have been triggered by a
malicious device.

Rip out this broken crap which is beyond repair instead of pretending
we support this feature.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>