mwifiex: refactor mwifiex_parse_htinfo() for reuse

This function converts some firmware-specific parameters into cfg80211
'rate_info' structures. It currently assumes it's dealing only with TX
bitrate, but the RX bitrate looks to be the same, so refactor this
function to be reusable.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>