rtlwifi: btcoex: 21a 2ant: check power save state before pstdma

The power_save_state function checks the state of power saving.
For tdma settings, the wifi sends nullfunc to pretend enter power saving
and then bt can transmit. Hence the coex needs to check the power status
before set the pstdma function.

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>