wifi: ath9k: use clamp() in ar9003_aic_cal_post_process()

When it needs to get a value within a certain interval, using clamp()
makes the code easier to understand than min(max()).

Signed-off-by: Li Zetao <lizetao1@huawei.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://patch.msgid.link/20240830011858.603514-1-lizetao1@huawei.com