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 >