ath9k: make ath_ps_ops structures as const

ath_ps_ops structures are only stored as a reference in the ps_ops
field of a ath_common structure. This field is of type const, so make
the structures as const.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>