[CPUFREQ] conservative: fix dbs_cpufreq_notifier so freq is not locked

When someone added the dbs_cpufreq_notifier section to the governor the
code ended up causing the frequency to only fall.  This is because
requested_freq is tinkered with and that should only modified if it has
an invlaid value due to changes in the available frequency ranges

This should fix #10055.

Signed-off-by: Alexander Clouter <alex@digriz.org.uk>
Signed-off-by: Dave Jones <davej@redhat.com>