spi: mediatek: fix spi incorrect endian usage

TX_ENDIAN/RX_ENDIAN bits define whether to reverse the endian
order of the data DMA from/to memory. The endian order should
keep the same with cpu endian.

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