iwlwifi: mvm: rs: use the proper channel width define for legacy rate

Use the 20Mhz channel width define instead of just the number
zero for legacy rates. Note that the define has the same value
so this is just a minor cleanup.

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