cpufreq: conservative: fix requested_freq reduction issue

When decreasing frequency, requested_freq may be less than
freq_target, So requested_freq minus freq_target may be negative,
But reqested_freq's unit is unsigned int, then the negative result
will be one larger interger which may be even higher than
requested_freq.

This patch is to fix such issue. when result becomes negative,
set requested_freq as the min value of policy.

Signed-off-by: Xiaoguang Chen <chenxg@marvell.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Git-commit: 3baa976ae644f76f5cdb5be0fb26754c3bfb32cb
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Junjie Wu <junjiew@codeaurora.org>