cm: etc: Mount BFQIO cgroup as /dev/bfqio

AOSP doesn't mount cgroups in /sys/fs/cgroups anymore.
Follow the convention and mount BFQIO cgroups in /dev/bfqio.

Also, remove the ioprio write as we only need ioprio_class
when dealing with 2 different priorities.

Change-Id: I8ff6c601547c445368e80832e736bb926d86b4d1