ath11k: pci: remove experimental warning

I have received feedback that QCA6390 PCI support is working for many, and I'm
also using QCA6390 on my daily driver^Hlaptop. While there are issues still
to be resolved it's not really experimental anymore, so remove the experimental
warning from driver initialisation.

Tested-on: QCA6390 hw2.0 PCI WLAN.HST.1.0.1-01740-QCAHSTSWPLZ_V2_TO_X86-1

Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1613040697-20289-1-git-send-email-kvalo@codeaurora.org