serial: imx: add error messages when .probe fails

Some functions called by serial_imx_probe emit an error message themself
(like kmalloc() and friends). clk_prepare_enable() and
devm_request_irq() however don't which might make the driver silently
fail to probe. So add an error message for these.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>