cpufreq: Return directly in __cpufreq_get if policy is NULL

__cpufreq_get() checks whether policy->cur matches frequency returned
by cpufreq device driver and acts accordingly. However, policy could
be NULL if the CPU is being hotplugged. Current implementation crashes
when trying to dereference the NULL policy.

Return the frequency provided by cpufreq device driver directly if
policy is not available.

Change-Id: I1f2ba4028c259392bf730db37dbec2d8c5ae3fa4
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>