ath5k: drop bogus warning on drv_set_key with unsupported cipher

commit a70e1d6fd6b5e1a81fa6171600942bee34f5128f upstream.

Simply return -EOPNOTSUPP instead.

Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>