cpufreq: powernv: fix missing check of return value in init_powernv_pstates()

kmalloc() could fail, so insert a check of its return value. And
if it fails, returns -ENOMEM.

And remove (struct pstate_idx_revmap_data *) to fix coccinelle WARNING
by the way.

WARNING: casting value returned by memory allocation function to (struct
pstate_idx_revmap_data *) is useless.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>