usb: dwc3-of-simple: fix error check of clk_get_bulk when disabled

The disabled clk API returns -ENOSYS unlike the reset API returning -ENOTSUPP.

Fixes: ca7fdc8b ("usb: host: Add simple of glue driver for DWC3 USB Controllers integration")
Reported-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>