rtlwifi: rtl8192de: Style clean-ups

Clean up some style issues:
- Use ARRAY_SIZE() even though it's a u8 array.
- Remove redundant CHANNEL_MAX_NUMBER_2G define.
Additionally fix some dead code WARNs.

Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Link: https://lore.kernel.org/lkml/57d0d1b6064342309f680f692192556c@realtek.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211119192233.1021063-1-keescook@chromium.org