ath11k: force single pdev only for QCA6390

For QCA6390, only one pdev is created and only one HW is registered to
mac80211. This one pdev manages both 2G radio and 5G radio.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1
Tested-on: IPQ8074 hw2.0 AHB WLAN.HK.2.1.0.1-01238-QCAHKSWPL_SILICONZ-2

Signed-off-by: Carl Huang <cjhuang@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1597576599-8857-10-git-send-email-kvalo@codeaurora.org