ARM: tegra: fix overflow in tegra20_pll_clk_round_rate()

32-bit math isn't enough when e.g. *prate=12000000, and sel->n=1000.
Use 64-bit math to prevent this.

Cc: Prashant Gaikwad <pgaikwad@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>