spi: davinci: set chip-select mode in SPIDEF only once

Quit writing the same constant value determining the chip-select
mode when no transmissions are in progress in davinci_spi_chipelect().

Instead just setup the SPIDEF register once during probe.

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