ath10k: fix debugfs_create_dir() checking

The function may return an -ENODEV if debugfs is
disabled in kernel. This should originally be
guarded by ath10k's Kconfig but it still makes
sense to check for the non-NULL errno return
value.

Reported-by: Matteo Croce <matteo@openwrt.org>
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>