mt76: mt7915: add vif check in mt7915_update_vif_beacon()

To avoid the warn_on() calltrace in station mode.

[ 4522.024382]  ieee80211_tx_dequeue+0x1258/0x1298 [mac80211]
[ 4522.029868]  ieee80211_beacon_get_template+0x10/0x18 [mac80211]
[ 4522.035780]  mt7915_mcu_add_beacon+0x2c/0x208 [mt7915e]
[ 4522.040997]  mt7915_mcu_get_rx_rate+0x304/0x878 [mt7915e]
[ 4522.046394]  ieee80211_delayed_tailroom_dec+0x158/0x180 [mac80211]
[ 4522.052573]  ieee80211_iterate_interfaces+0x48/0x68 [mac80211]
[ 4522.058398]  mt7915_mac_reset_work+0x3b4/0x630 [mt7915e]
[ 4522.063704]  process_one_work+0x1fc/0x390
[ 4522.067703]  worker_thread+0x48/0x4d0
[ 4522.071356]  kthread+0x120/0x128

Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>