spi: davinci: remove unnecessary disable of SPI

In the davinci_spi_bufs_dma() function, SPI is briefly disabled
before enabling it immediately back again.

Remove this unnecessary disable.

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