spi: mediatek: remove clk_disable_unprepare()

This patch removes clk_disable_unprepare() in mtk_spi_remove().
clk_disable_prepare/unprepare must be balance, spi-clk is disabled
in mtk_spi_probe, so not needs to disable again.

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