wifi: ath9k: fix parameter check in ath9k_init_debug()

Make IS_ERR() judge the debugfs_create_dir() function return
in ath9k_init_debug()

Signed-off-by: Minjie Du <duminjie@vivo.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230712114740.13226-1-duminjie@vivo.com