rtw88: add path diversity

This feature chooses to transmit with antenna that has better signal
strength periodically under 1ss rate.

It can benefit connection quality in the following cases:
1. User is far away from the AP.
2. The far-field pattern of the antenna showed significant signal
strength difference.

Signed-off-by: Po-Hao Huang <phhuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210426013252.5665-2-pkshih@realtek.com