cpufreq: brcmstb-avs-cpufreq: Fix types for voltage/frequency

What we read back from the register is going to be capped at 32-bits,
and cpufreq_freq_table.frequency is an unsigned int. Avoid any possible
value truncation by using the appropriate return value.

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>