cpufreq: brcmstb-avs-cpufreq: ISO C90 forbids mixed declarations

There is a compile warning because a NULL pointer check was added before
a struct was declared. This moves the NULL pointer check to after the
struct is declared and moves the struct assignment to after the NULL
pointer check.

Fixes: f661017e ("cpufreq: brcmstb-avs-cpufreq: add check for cpufreq_cpu_get's return value")
Signed-off-by: Portia Stephens <portia.stephens@canonical.com>
Acked-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>