rtlwifi: btcoex: 21a 2ant: refine tdma duration adjust function

1. Add more cases to adjust the wifi duration and add a case with
the max interval of 3 for some future uses.

2. rename tdma_adj -> ps_tdma_du_adj to indicate that this member is
for power saving tdma duration adjustment

Signed-off-by: Yan-Hsuan Chuang <yhchuang@realtek.com>
Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Pkshih <pkshih@realtek.com>
Cc: Birming Chiu <birming@realtek.com>
Cc: Shaofu <shaofu@realtek.com>
Cc: Steven Ting <steventing@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>