b43: Add optional verbose runtime statistics

This adds support for verbose runtime statistics.
It defaults to off and must be enabled in debugfs, if desired.
The first measurement may be incorrect, because statistics are not cleared
after they got enabled through debugfs.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>