wifi: ath12k: enable WIPHY_FLAG_DISABLE_WEXT

ath12k will support Multi-Link Operation (MLO) in the future but not yet. A
major change with MLO is that cfg80211 will disable Wireless Extension (WEXT)
user space interface. To avoid having inconsistent user experience, first
supporting WEXT and later not, disable WEXT already now.

Now ath12k claims that WEXT is not supported:

$ iwconfig wlan0
wlan0     no wireless extensions.

Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Acked-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Link: https://msgid.link/20240411165645.4071238-1-kvalo@kernel.org