cpufreq: imx6q: save one condition block for normal case of nvmem read

Put return value checks of calling imx6ul_opp_check_speed_grading()
into one block to save one condition block for normal case.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>