rtlwifi: btcoex: Use seq_file to dump btcoex status

We use seq_file to replace RT_TRACE to dump status, then we can use 'cat'
to access btcoex's status through debugfs.
(i.e. /sys/kernel/debug/rtlwifi/00-11-22-33-44-55-66/btcoex)
Other related changes are
1. implement btc_disp_dbg_msg() to access btcoex's common status.
2. remove obsolete field bt_exist

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