rtl8xxxu: feed antenna information for cfg80211

Fill up the available TX/RX antenna so the iw commands can show
correct antenna information for different chips.

Signed-off-by: Chris Chiu <chris.chiu@canonical.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220325035735.4745-2-chris.chiu@canonical.com