wifi: ath9k: fix parameter check in ath9k_init_debug()

wifi: ath9k: fix parameter check in ath9k_init_debug()

[ Upstream commit 6edb4ba6fb5b946d112259f54f4657f82eb71e89 ]

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
Stable-dep-of: f6ffe7f01847 ("wifi: ath9k: Remove error checks when creating debugfs entries")
Signed-off-by: Sasha Levin <sashal@kernel.org>