wl18xx: number_of_assembled_ant5 indicates if A band is enabled

Use number_of_assembled_ant5 phy param to indicate if A band is enabled:
if number_of_assembled_ant5 != 0 then it is enabled otherwise it is
disabled.

This aligns with phy implementation that uses this param both to indicate
if band is active and the number of antennas.

This parameter replaces enable_11a module param that was removed.

User-Space applications can use wlconf and/or INI files to disable A band
using this parameter.

Signed-off-by: Yair Shapira <yair.shapira@ti.com>
Signed-off-by: Igal Chernobelsky <igalc@ti.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>