iwlwifi: mvm: slightly clean up rs_fw_set_supp_rates()

The "supp" variable doesn't need to be unsigned long, only
"tmp" is used with for_each_set_bit(). "supp" should just
be a u16, since that's how it's sent to the firmware.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210205110447.762e50704a39.I014bc7898f90c734f8e9be2a3efaf9bf8b7db6db@changeid
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>