cpufreq: brcmstb-avs-cpufreq: Fix initial command check

There is a logical error in brcm_avs_is_firmware_loaded() whereby if the
firmware returns -EINVAL, we will be reporting this as an error. The
comment is correct, the code was not.

Fixes: de322e08

 ("cpufreq: brcmstb-avs-cpufreq: AVS CPUfreq driver for Broadcom STB SoCs")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>