if_dwc: Ignore clk_set_assigned errors if "assigned_clocks" is not specified

dwc_attach will currently fail if clk_set_assigned fails, even if the
"assigned_clock" property was not specified. Since there are platforms
whose DTS files do not have this property (e.g. if_dwc_aw,
if_dwc_socfpga), do not return an error if clk_set_assigned
returns ENOENT.

Reviewed by: manu
Differential review: https://reviews.freebsd.org/D48529