b43: warn when forcing PIO mode

We have resolved all the known issues with DMA mode, however some users
(or distros) are still forcing PIO mode by config files. Without
debugging enabled it's not noticable at all. Add the warning for them.

Cc: Gregory Bellier <gregory.bellier@gmail.com>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>