mwifiex: don't print error message on coex event

The BT coex event is not an error condition. Don't print an error
message in this case. The same even in sta_event.c prints a
message using the debug level already.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>