rtw89: coex: Define BT B1 slot length

It is for setting up BT slot max length at BT auto slot mechanism.

Signed-off-by: Ching-Te Ku <ku920601@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220725023509.43114-5-pkshih@realtek.com