spi/tegra: remove checks for valid speed

SPI core make sure that all transfer has proper speed set
before calling low level spi transfer. Hence, it is not
require to have check in spi driver.

Remove the check for speed validity from transfer and use it directly.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>