wl18xx: add module parameter to force SISO 20MHz

In some cases it may be useful to force narrow-band SISO channels.
Add a new value to the ht_mode module parameter to force the device to
operate in SISO 20MHz.

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