rtw88: coex: Set 4 slot mode for A2DP

With shallow buffer size, certain BT devices have active
A2DP flow control to fill buffer frequently. If the slot
is not at BT side, data can't be sent successfully to BT
devices, and will cause audio glitch.

To resolve this issue, this commit splits TUs into 4-slots
instead of 2-slot for all of the A2DP related coexistence
strategies. That makes BT have higher opportunity to fill
the A2DP buffer in time, and the audio quality could be
more stable and smooth.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>