soc/tegra: pmc: Guard against uninitialised PMC clock

It is possible for the public functions, tegra_io_rail_power_on/off()
to be called before the PMC device has been probed. If this happens
then the pmc->clk member will not be initialised and the call to
clk_get_rate() in tegra_io_rail_prepare() will return zero and lead
to a divide-by-zero exception. The function clk_get_rate() will return
zero if a NULl clk pointer is passed. Therefore, rather that checking
if pmc->clk is initialised, fix this by checking the return value for
clk_get_rate() to make sure it is not zero.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>