spi: davinci: configure the invariable bits in spipc0 only once

Configure the data-in, data-out and clock functionality pins
in SPIPC0 register only once during probe. No need to set these
bits for each transfer.

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