spi: davinci: remove unnecessary completion variable initialization

The completion variable 'done' is unnecessarly initialized by
the function davinci_spi_bufs_dma() where as it is not used
for DMA transfers at all.

Remove the unnecessary initialization.

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