iwlwifi: mvm: debugfs: add an option to set antennas for scan command

Add an option to set rx antennas for the scan command from debugfs.
Create a file called ant_rxchain in the mvm debugfs directory.
To choose antennas, write a number between 1-7 to ant_rxchain.
Write 1 for A, 2 for B, 3 for AB and so on.

Signed-off-by: Oren Givon <oren.givon@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>