PowerHAL: Update the hispeed_freq value before using it

The hispeed_freq value is read only in the inizialization process.
If we decide to change it later writing a new value to the node
this new value wont be taken into consideration by the power hal.
Lets update the hispeed_freq value in the power hal before using
it to limit the max cpu freq when we go into power save mode.

Change-Id: I4b8c7f67259750b2931b18c3871874022aa6054d