When updating the CPU PLL frequency, keeping the PLL enabled avoids
ramping the PLL all the way down and back up again.  Remove the BUG_ON
in tegra2_pll_clk_set_rate to allow the rate to change while the PLL
is enabled.
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Colin Cross <ccross@android.com>