pinctrl: rzn1: Fix of_get_child_count() error check

If we assign the result of of_get_child_count() to an unsigned int,
the code will not detect any errors. Therefore assign it to an int
instead.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>