ath9k: split set11nRateFlags and set11nChainSel

This makes the code clearer since set11nRateFlags currently sets
both the rate flags and chain sel. This may also be required for
QCN550x support, where the rate flags and chain sel are in separate
fields.

This change does not appear to affect the final binary.

Signed-off-by: Wenli Looi <wlooi@ucalgary.ca>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220320233010.123106-3-wlooi@ucalgary.ca