soc: renesas: rcar-sysc: Fix uninitialized error code in rcar_sysc_pd_init()

On success, rcar_sysc_pd_init() returns an uninitialized error code.
Use the return value of of_genpd_add_provider_onecell() to fix this.

This went unnoticed, as early_initcall() doesn't care about the return
value.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>