iwlwifi: mvm: add support for single antenna diversity

There are products which have a single chain with 2 antennas.
In these products, we need to inform the FW that the device has the
single antenna diversity(SAD) feature. In the future, we will read
the active antenna from a BIOS configuration. Currently, we use a
default configuration which means that the FW decides which antenna to use.

Signed-off-by: Haim Dreyfuss <haim.dreyfuss@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>