spi: tegra: don't treat NULL clk as an error

Some platforms have been known to return NULL from clk_get() if they
support only a single struct clk.  Whilst tegra doesn't do this, make
the drivers consistent with others.

Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Acked-by: Russell King <linux@arm.linux.org.uk>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>