serial: atmel: fix error path of probe function

If atmel_init_gpios fails the port has already been marked as busy (in
line 2629), so this must be undone in the error path.

This bug was introduced because I created the patch that finally
became 722ccf41 ("serial: atmel: fix error handling when
mctrl_gpio_init fails") on top of 3.19 which didn't have commit
6fbb9bdf ("tty/serial: at91: fix error handling in
atmel_serial_probe()") yet.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Fixes: 722ccf41 ("serial: atmel: fix error handling when mctrl_gpio_init fails")
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>