spi: davinci: remove unused members of davinci_spi_slave

Several members of struct davinci_spi_slave are unused in code.
Remove such members.

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