cpufreq: scmi: Avoid overflow of target_freq in fast switch

Conversion of target_freq to HZ in scmi_cpufreq_fast_switch()
can lead to overflow if the multiplied result is greater than
UINT_MAX, since type of target_freq is unsigned int. Avoid this
overflow by assigning target_freq to unsigned long variable for
converting it to HZ.

Signed-off-by: Jagadeesh Kona <quic_jkona@quicinc.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>