rtlwifi: 8821a: btcoexist: add comments to explain why if-else branches are identical

The coexistence programmers preserve the same code of branches
intentionally to fine tune performance easier, because bandwidth and RSSI
strength are highly related to coexistence performance. The basic rule of
performance tuning is to assign most time slot to BT for realtime
application, and WiFi uses remaining time slot but don't lower than low
bound.

Reported-by: Inigo Huguet <ihuguet@redhat.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210506013738.5943-1-pkshih@realtek.com