ath10k: Change the warning message string

The "WARNING" string confuses syzbot, which thinks it found
a crash [1].

Change the string to avoid such problem.

[1] https://lkml.org/lkml/2019/5/9/243



Reported-by:  <syzbot+c1b25598aa60dcd47e78@syzkaller.appspotmail.com>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>