wl18xx: add a module parameter to control 11a support

In wl18xx we don't have an NVS yet, so we need to control whether 11a
is supported with a module parameter for now.  11a support is enabled
by default and can be changed by setting enable_11a to false when
loading the module.

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