spi: omap2-mcspi: Do not configure the controller on each transfer unless needed

spi_transfer->speed_hz and spi_transfer->bits_per_word used to only be set when
not using the default settings but are not set on every transfer, causing
omap2_mcspi_setup_transfer to be called on each transfer. This patch changes
the check to only call omap2_mcspi_setup_transfer if the settings needs to be
changed.

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: Mark Brown <broonie@linaro.org>