b43: Add NPHY kconfig option

This adds a new Kconfig option for enabling probing of N-PHYs.
This option will be removed again once the stuff works.
For now it is to help in development. This way real users won't
execute the broken N-PHY codepaths, but the developers can easily
enable N-PHY stuff.

To enable N-PHY probing simply remove the BROKEN dependency
and enable the option in the kernel config.

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