wifi: iwlwifi: mvm: rename iwl_mvm_mac80211_idx_to_hwrate()

wifi: iwlwifi: mvm: rename iwl_mvm_mac80211_idx_to_hwrate()

Given that we now use v3 rates with FW index throughout,
_to_hwrate() is confusing, since the hardware still uses
the PLCP value, the driver just doesn't see that now (as
it talks to firmware, not hardware.)

Rename this to iwl_mvm_rate_idx_to_fw_idx() to more
clearly indicate what it's doing.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://patch.msgid.link/20260527230313.a60c8aea5b6c.I6af48d5d9748e184eed9d3437d312291cab61d7f@changeid
Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>