MLK-9986-2 spi: spi-imx: use pio mode for the tail data in DMA mode

Sometimes the tail data can't trigger SDMA to read from rxfifo, or
SDMA miss the last dma request, in this case, DMA report RX timeout
and the rest tail data kept in rxfifo. Whatever, use pio read for the
tail rx data.

Signed-off-by: Robin Gong <b38343@freescale.com>
(cherry picked from commit 61fec9e4511e7626a2c5299f37c12cca95d12e88)
(cherry picked from commit c7ecc24637babcb76edf20600f0535ab118e0101)