cpufreq: omap: use cpufreq_generic_init()

Use generic cpufreq_generic_init() routine instead of replicating the same code
here.

This also rearranges the code a bit to make it more sensible. Also removes some
unnecessary checks.

Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>