cpufreq: scpi-cpufreq: signedness bug in scpi_get_dvfs_info()

The "domain" variable needs to be signed for the error handling to work.

Fixes: 8def3103

 (cpufreq: arm_big_little: add SCPI interface driver)
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>