iwlwifi: mvm: rs: remove the ANT C from the toogle antenna logic

We don't plan to have products with 3 antennas in the near
future. All the rest of the code follows the same
assumption as well.
Remove the support for antenna C from rs_toggle_ant.
When trying to toggle from ANT_B, this avoids to go through
ANT_C, discover that it doesn't exist and continue to ANT_A.
In MIMO, this avoids to do ANT_AB -> ANT_BC -> ANT_AC and
back to ANT_AB.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>