b43: Increase loop tries in do_dummy_tx

One of the spin-on-condition loops in routine do_dummy_tx always exits before
the condition is satisfied. The hardware might be left in an inconsistent
state that might be the cause of the PHY transmission errors seen by some
users.

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