iwlwifi: mvm: rs: avoid recalc of supported legacy rate mask

The supported legacy rate mask is initialized when rs
is initialized based on the remote peer supported rates.
There's no need to re mask it repeatedly with the supported
remote peer rates.

Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>