wl18xx: add support for ht_mode in conf.h

ht_mode added to wl18xx conf struct in order to support different modes
from the configuration file, as well as module params, and by default
(working without a conf file and/or no module params).
the hack regarding conf.phy.low_band_component_type for each board
is now explicitly handled after parsing module params.
missing default values to wl18xx config added.
fix string module params not to have defaults (so if empty, param
can be taken from conf file).
update conf version to 3.

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