sched: do not return error when set the same sched_boost value

Only return error when switching from one boost type to another.

Bug: 80322824
Test: no error set same value to /proc/sys/kernel/sched_boost several
      times
Test: error when set "1","2" to /proc/sys/kernel/sched_boost alternately
      to change mode without set "0" first
Signed-off-by: Wei Wang <wvw@google.com>
Change-Id: Id6b291cfd932e1f7f69b519edcc8e7e57df569b1
Signed-off-by: improbir <ghosh_probir@yahoo.com>