Fix a lockup in tx path for cspw.

Sometimes the software loses the race when appending more descriptors to
the tx ring and the tx queue stops.

This commit detects this condition and restart the tx queue whenever it stall.

Tested by: sobomax@, Keith White <kwhite@site.uottawa.ca>,
Paul Mather <paul@gromit.dlib.vt.edu>
Sponsored by: Rubicon Communications (Netgate)
Approved by: re (kib)