spi: tegra20-sflash: Convert to use master->max_speed_hz

Use master->max_speed_hz instead of tspi->spi_max_frequency, so spi core will
handle checking transfer speed.

In additional, since commit 052eb2d4 'spi: core: Set max_speed_hz of
spi_device default to max_speed_hz of controller',
spi core will also set default spi->max_speed_hz if it is not set.
So remove tegra_sflash_setup().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>