[B43]: Support for turning the radio off from software.

This adds support for turning the radio off in software.
That's useful in environments, where you don't want the RF
to radiate any signals, but don't want to bring the interface down.

Cc: Larry Finger <larry.finger@lwfinger.net>
Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>