wifi: mt76: mt7921: do not support one stream on secondary antenna only

mt7921 support following antenna combiantions only.
* primary + secondary (2x2)
* primary only        (1x1)

Since we cannot work on secondary antenna only, return error if the
antenna bitmap is 0x2 in .set_antenna().

For example:
iw phy0 set antenna 3 3 /* valid */
iw phy0 set antenna 1 1 /* valid */
iw phy0 set antenna 2 2 /* invalid */

Cc: stable@vger.kernel.org
Fixes: e0f9fdda

 ("mt76: mt7921: add ieee80211_ops")
Suggested-by: Knox Chiou <knoxchiou@google.com>
Signed-off-by: Deren Wu <deren.wu@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>