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>

drivers/spi/spi-mt65xx.c

@@ -522,11 +522,11 @@ static irqreturn_t mtk_spi_interrupt(int irq, void *dev_id)
 		mdata->xfer_len = min(MTK_SPI_MAX_FIFO_SIZE, len);
 		mtk_spi_setup_packet(master);
 
-		cnt = len / 4;
+		cnt = mdata->xfer_len / 4;
 		iowrite32_rep(mdata->base + SPI_TX_DATA_REG,
 				trans->tx_buf + mdata->num_xfered, cnt);
 
-		remainder = len % 4;
+		remainder = mdata->xfer_len % 4;
 		if (remainder > 0) {
 			reg_val = 0;
 			memcpy(&reg_val,