b43legacy: Rewrite pwork locking

Implement much easier and more lightweight locking for
the periodic work. This also removes the last big busywait
loop and replaces it by a sleeping loop.

This patch for b43legacy is patterned aftar the same patch
for b43 by Michael Buesch <mb@bu3sch.de>.

Signed-off-by: Larry Finger <larry.finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>