spi: omap3: pre-initialize bus-speed with max. slave-speed

Otherwise the frequency is zero and the clock divider cannot be setup by
'omap3_spi_set_speed' function.

Signed-off-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Reviewed-by: Jagan Teki <jagan@openedev.com>