b43: Issue warning when RFKILL_INPUT is not enabled

If the system is misconfigured with CONFIG_RFKILL set but CONFIG_RFKILL_INPUT
not set, the built-in radio LEDs will not work. In the current code, no warning
is issued.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>