ath9k: Incorrect key used when group and pairwise ciphers are different.

Updating sc_keytype multiple times when groupwise and pairwise
ciphers are different results in incorrect pairwise key type
assumed for TX control and normal ping fails. This works fine
for cases where both groupwise and pairwise ciphers are same.

Also use mac80211 provided enums for key length calculation.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>