spi: mediatek: remove mtk_spi_config

mtk_spi_config() and mtk_spi_prepare_message() both initialize
spi register, so remove mtk_spi_config() and init all register
in mtk_spi_prepare_message().

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