ath9k: use AR9300_MAX_CHAINS when appropriate

Replace other constants with AR9300_MAX_CHAINS when appropriate.

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/20220320233010.123106-4-wlooi@ucalgary.ca