spi: tegra: do not use clock name to get clock

Since Tegra spi devices do not have multiple clocks, no need to use
clock name to get the clock.

Signed-off-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Stephen Warren <swarren@nvidia.com>