wlcore: add plt_mode including new PLT_FEM_DETECT

add wl->plt_mode that is used to indicate different plt
working modes: this will be used to implement calibrator side
auto fem detection where driver asks firmware to detect
the wlan fem radio type and returns it to calibrator.

this is not implemented yet and plt_modes: PLT_ON and
PLT_FEM_DETECT currently behave the same.

Signed-off-by: Yair Shapira <yair.shapira@ti.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>