wl1271: Add sysfs file to control BT co-ex state

Add a sysfs file to control the state of the BT co-ex (enable or disable it.)
By default, the BT co-ex is enabled.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Reviewed-by: Teemu Paasikivi <ext-teemu.3.paasikivi@nokia.com>
Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>