spi: davinci: handle DMA completion errors correctly

Do not simply clean the DMA channel on a DMA completion
error. Instead, use wcount and rcount members of davinci_spi
to detecion non-completion of DMA and signal EIO to the
application.

Signed-off-by: Brian Niebuhr <bniebuhr@efjohnson.com>
Tested-By: Michael Williamson <michael.williamson@criticallink.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>