rtw88: coex: upgrade coexistence A2DP mechanism

Add modifications for A2DP interoperability issue of TWS earphones.
Some TWS earphones has low buffer size, so it's necessary to reduce
the slot period to let it get data more frequently.
(e.g. use 4-slot to replace the original 2-slot method).

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20201126021059.11981-10-pkshih@realtek.com