spi: omap2-mcspi: Undo broken fix for dma transfer of vmalloced buffer

This reverts commit 3525e0aa

.

The DMA transfer for RX buffer was not handled correctly in this change.

The actual transfer length for DMA RX can be less than xfer->len in the
specific condition and the last words will be filled after the DMA
completion, but the commit doesn't consider it and the dmaengine is
started with rx_sg mapped by spi core.

The solution for this at least requires more lines than this commit
has inserted.  So revert it for now.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>