spi: make sure all transfer has proper speed set

When spi client does the spi transfer and if it does not set
the speed for each transfer then set it as default
of spi device in spi core before calling low level transfer.

This will remove the extra check in low level driver for setting
speed.

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