wifi: ath9k: Use swap() to improve ath9k_hw_get_nf_hist_mid()

Use the swap() macro to simplify the ath9k_hw_get_nf_hist_mid() function
and improve its readability.

Fixes the following Coccinelle/coccicheck warning reported by
swap.cocci:

  WARNING opportunity for swap()

Compile-tested only.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://patch.msgid.link/20240710185743.709742-2-thorsten.blum@toblux.com