clk: tegra: Init cfg structure in _get_pll_mnp

Not all fields are read from the hw depending on the PLL type. Make sure
the other fields are 0 by clearing the structure beforehand to prevent
users such as the rate re-calculation code from using bogus values.

Based on work by  Alex Frid <afrid@nvidia.com>

Signed-off-by: Peter De Schrijver <pdeschrijver@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>