spi: omap2_mcspi rxdma bugfix

When data is read through DMA, the last element must be read separately
through the RX register.  It cannot be transferred by the DMA.  For
further details see e.g.  OMAP35x TRM (table 19-16).

Without the fix the driver causes extra clocks to be clocked to the bus
after DMA RX operations.  This can cause interesting behaviour with some
devices.

Signed-off-by: Juuso Oikarinen <juuso.oikarinen@nokia.com>
Signed-off-by: Eero Nurkkala <ext-eero.nurkkala@nokia.com>
[aaro.koskinen@nokia.com: Simplified the patch while keeping the idea.]
Signed-off-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>