wifi: iwlwifi: mvm: refactor iwl_mvm_set_sta_rate() a bit

Use a switch statement over the rate type instead of the
if chain, to simplify.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20220906161827.623e32931b67.Id743a7879e84ae37a849179e536c58b1bf55380f@changeid


Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>