Make sure to check whether driver is running before processing

Make sure to check whether driver is running before processing
received frames. Also check driver has valid ifp pointer before
calling msk_stop() in device_shutdown handler. While I'm here
remove unnecessary accesses to interrupt mask registers in
device_shutdown handler because driver puts the controller into
reset state.
With these changes, msk(4) now survive from heavy RX traffic(1byte
UDP frame) while reboot is in progress.

Reported by:	Mark Atkinson < atkin901 <> gmail dot com >