PM / devfreq: tegra30: Don't enable already enabled consecutive interrupts

Consecutive up/down interrupt-bit is set in the interrupt status register
only if that interrupt was previously enabled. Thus enabling the already
enabled interrupt doesn't do much for us.

Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>