PM / OPP: -ENOSYS is applicable only to syscalls

Some of the routines have used -ENOSYS for the cases where the
functionality isn't implemented in the kernel. But ENOSYS is supposed to
be used only for syscalls.

Replace that with -ENOTSUPP, which specifically means that the operation
isn't supported.

While at it, replace exiting -EINVAL errors for similar cases to
-ENOTSUPP.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>