Simplify checking for unsupported bus speeds and reporting errors by
factoring out the calculation of the maximum bus speed, and by using the
dev_err_probe() helper.

While at it, use "%u" for u32, and improve the error message.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Tested-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Andi Shyti <andi.shyti@kernel.org>