ath11k: change to treat alpha code na as world wide regdomain

Some firmware versions for WCN6855 report the default regdomain with
alpha code "na" by default when load as a world wide regdomain, ath11k
should treat it as a world wide alpha code.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-01720.1-QCAHSPSWPL_V1_V2_SILICONZ_LITE-1

Signed-off-by: Wen Gong <quic_wgong@quicinc.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211118094848.7776-1-quic_wgong@quicinc.com