serial: imx: fix error handling in console_setup

The ipg clock only needs to be unprepared in case preparing
per clock fails. The ipg clock has already disabled at the point.

Fixes: 1cf93e0d ("serial: imx: remove the uart_console() check")
Signed-off-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>