wl18xx: sane defaults for HT capabilities

Introduce a default set of HT capabilities that are set according to the
number of antennas on the board. Move the HT setting code down to allow
the number of antennas to be set (and optionally overridden) before it.

Remove the "mimo" HT option, since the default mode now enables MIMO is
possible.

Use this opportunity to add a helper function for setting HT
capabilities and reduce the volume of the code a bit.

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