iwlwifi: fix iwl_legacy_rate_to_fw_idx

There are a couple of bugs in this function:

1. It is declared as a non-static function, even though
   it's only used in one file.
2. Its return value should be of type u32 but it returns
   (in some cases) -1.

Fix them by making this function static and returning an
error value of type unsigned.

In addition, we're assigning the return value of this function
as the legacy rate even if the function returned an error value.
Fix this by assigning the lowest rate in this case.

Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Reported-by: Ye Guojin <ye.guojin@zte.com.cn>
Reported-by: Zeal Robot <zealci@zte.com.cn>
Fixes: 9998f81e ("iwlwifi: mvm: convert old rate & flags to the new format.")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/iwlwifi.20220128142706.5612eeb9d6d0.I992e10d93fc22919b2bc42daad087ee1b5d6f014@changeid