ath10k: ATH10K_DEBUGFS depends on DEBUG_FS

ATH10K_DEBUGFS must depend on DEBUG_FS, otherwise
ath10k will generate an invalid pointer on module load

Signed-off-by: Matteo Croce <matteo@openwrt.org>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>