wifi: ath12k: Introduce and use ath12k_sta_to_arsta()

Currently, the logic to return an ath12k_sta pointer, given a
ieee80211_sta pointer, uses typecasting throughout the driver. In
general, conversion functions are preferable to typecasting since
using a conversion function allows the compiler to validate the types
of both the input and output parameters.

ath12k already defines a conversion function ath12k_vif_to_arvif() for
a similar conversion. So introduce ath12k_sta_to_arsta() for this use
case, and convert all of the existing typecasting to use this
function.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20231019-upstream-ath12k_sta_to_arsta-v1-1-06f06f693338@quicinc.com