ARM: tegra: clock: prevent accidental disables of cpu clock

Peripheral clocks that have no clock enable bit in the
enable registers have their clk_num set to 0.  Bit 0
in the clock enable registers is the CPU clock.
Prevent disables on these peripheral clocks from
accidentally disabling the CPU clock.

Signed-off-by: Colin Cross <ccross@android.com>
Acked-by: Olof Johansson <olof@lixom.net>