tegra: Correct PLL access in ap20.c and clock.c

tegra: Correct PLL access in ap20.c and clock.c

Correct this warning seen by Albert:

ap20.c:44:18: warning: array subscript is above array bounds

There is a subtle bug here which currently causes no errors, but might
in future if people use PCI or the 32KHz clock. So take the opportunity
to correct the logic now.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>