ath11k: delay vdev_start for QCA6390

For QCA6390 firmware, bss peer must be created before vdev_start, so delay
vdev_start until bss peer is created.

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/1597555891-26112-6-git-send-email-kvalo@codeaurora.org