mac80211: do not pass injected frames without a valid rate to the driver

Fall back to rate control if the requested bitrate was not found.

Fixes: dfdfc2be

 ("mac80211: Parse legacy and HT rate in injected frames")
Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>