mt76: add API for testmode support

This can be used for calibration in the manufacturing process.
It supports sending a configurable number of packets with a specific rate
and configurable tx power levels / antenna settings.
It also supports receiving packets and showing some statistics, including
packet counters and detailed RSSI information.
It will only be compiled in if CONFIG_NL80211_TESTMODE is enabled

Signed-off-by: Felix Fietkau <nbd@nbd.name>