clk: tegra: Use definition for pll_u override bit

The definition, PLLU_BASE_OVERRIDE, for the pll_u OVERRIDE bit is defined
but not used and when the OVERRIDE bit is cleared in tegra210_pll_init()
the code directly uses the bit number. Therefore, use the definition,
PLLU_BASE_OVERRIDE when clearing the OVERRIDE bit.

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