wcn36xx: Add accessor macro HW_VALUE_CHANNEL for hardware channels

Adds HW_VALUE_CHANNEL(hw_value) an access macro that will be used to
extract the channel number from struct ieee80211_channel->hw_value in
preparation for also storing PHY settings for 802.11ac in the upper bits of
hw_value.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200910150708.2179043-2-bryan.odonoghue@linaro.org