spi: davinci: remove unnecessary typecast

The typecasting of SPI base address to davinci_spi_reg is
unused.

Remove it.

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