greybus: svc: fix pwrmon return value

Errno -ENOSYS is reserved for missing syscalls, replace it with ENOMSG.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>