mt76: mt7615: fall back to sw encryption for unsupported ciphers

mt76: mt7615: fall back to sw encryption for unsupported ciphers

Fix following warning falling back to sw encryption for unsupported
ciphers

WARNING: CPU: 2 PID: 1495 at backports-4.19.32-1/net/mac80211/key.c:1023
mt76_wcid_key_setup+0x68/0xbc [mt76]
CPU: 2 PID: 1495 Comm: hostapd Not tainted 4.14.131 #0
Stack : 00000000 8f0f8bc0 00000000 8007ccec 805f0000 8058ec18 00000000 00000000
	80559788 8dca79bc 8fefb10c 805c89c7 805545c8 00000001 8dca7960 53261662
	00000000 00000000 80640000 00004668 00000000 000000e9 00000007 00000000
	00000000 805d0000 00072537 00000000 80000000 00000000 805f0000 8f1e70d0
	8e8fa098 000003ff 805c0000 8f0f8bc0 00000001 802d4340 00000008 80630008
[<800108d0>] show_stack+0x58/0x100
[<8049214c>] dump_stack+0x9c/0xe0
[<80033998>] __warn+0xe0/0x138
[<80033a80>] warn_slowpath_null+0x1c/0x2c
[<8e8fa098>] mt76_wcid_key_setup+0x68/0xbc [mt76]
[<8e889930>] mt7615_eeprom_init+0x7c0/0xe14 [mt7615e]

Suggested-by: Sebastian Gottschall <s.gottschall@newmedia-net.de>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>