ath10k-ct: fix module depends

ath10k-ct: fix module depends

PKG_EXTMOD_SUBDIRS should be set to the sub directory where the kernel
module gets build in, for the ath10k-ct driver this changed in commit
3888e77c1c68 from ath10k to ath10k-4.13. Without this fix the depends
line of the ath10*.ko modules is empty and the kernel module load system
will not automatically load the depended modules like mac80211.

Fixes: 3888e77c1c68 ("ath10k-ct driver: use dma_alloc_coherent, 4.13 based driver")
Fixes: 23a388fe41ec ("ath10k-ct: Force loading mac80211 and ath modules.")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>