serial: 8250_of: fix return code when probe function fails to get reset

The error pointer from devm_reset_control_get_optional_shared() is
not propagated.

One of the most common problem scenarios is it returns -EPROBE_DEFER
when the reset controller has not probed yet.  In this case, the
probe of the reset consumer should be deferred.

Fixes: e2860e1f ("serial: 8250_of: Add reset support")
Cc: stable@vger.kernel.org # v4.13+
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>