brcmsmac: don't start device when RfKill is engaged

This patch fixes a bug when device is being started
while RfKill switch is engaged, leading to hang
due to partial initialization of hardware.

Tested-by:  <dragonn@op.pl>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>