wlcore/wl18xx/wl12xx: separate channel count between chips

18xx chips are capable of staying on 2 channels at the same time.
Introduce a chip-family specific parameter to set the number of channels
in the interface-combinations published by the driver.

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>