wifi: ath9k: fix ath9k_use_msi declaration

Sparse warns:

drivers/net/wireless/ath/ath9k/init.c:79:5: warning: symbol 'ath9k_use_msi' was not declared. Should it be static?

Move the extern to ath9k.h so that it's visible in init.c.

Compile tested only.

Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://msgid.link/20240320170656.3534265-3-kvalo@kernel.org