wifi: ath9k: use IS_ERR() with debugfs_create_dir()

The debugfs_create_dir() function returns error pointers,
it never returns NULL. Most incorrect error checks were fixed,
but the one in ath9k_htc_init_debug() was forgotten.

Fix the remaining error check.

Fixes: e5facc75 ("ath9k_htc: Cleanup HTC debugfs")
Signed-off-by: Wang Ming <machel@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/20230713030358.12379-1-machel@vivo.com