ath11k: enable 6G channels for WCN6855

For some chips such as WCN6855, single_pdev_only is set in struct
ath11k_hw_params which means ath11k calls ieee80211_register_hw() only
once and create only one device interface, and that device interface
supports all 2G/5G/6G channels.

ath11k_mac_setup_channels_rates() sets up the channels and it is called
for each device interface. It is called only once for single_pdev_only,
and then set up all channels for 2G/5G/6G. The logic of
ath11k_mac_setup_channels_rates() is not suitable for single_pdev_only,
it leads to all 6G channels being disabled for the device interface
which is single_pdev_only such as WCN6855.

Add channel frequency checks for the 6G band and enable the 6G channels
properly based on what is supported by the chip.

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

Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Jouni Malinen <jouni@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210804181217.88751-3-jouni@codeaurora.org