PM / OPP: handle allocation of device_opp in a separate routine

Get the 'device_opp' allocation code into a separate routine to keep only the
necessary part in dev_pm_opp_add_dynamic().

Also do s/sizeof(struct device_opp)/sizeof(*dev_opp) and remove the print
message on kzalloc() failure as checkpatch warns for that.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
(cherry picked from commit 07cce74a7b259cb90029530e9549bf1d9a1b1c34)
Signed-off-by: Alex Shi <alex.shi@linaro.org>