ath11k: disable regdb support for QCA6390

Currently it does not have regdb files for QCA6390, so disable its regdb
support feature now.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-05266-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220319023543.14288-3-quic_wgong@quicinc.com