spi: mediatek: revise mtk_spi_probe() failure flow

mtk_spi_probe() calls pm_runtime_enable(), after
pm_runtime_enable() is called, it should call
pm_runtime_disable() in the failure flow.

Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>