spi: mediatek: use correct mata->xfer_len when in fifo transfer

when xfer_len is greater than 64 bytes and use fifo mode
to transfer, the actual length from the third time is mata->xfer_len
but not len in mtk_spi_interrupt().

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