usb: dwc3: Use ASCII space in Kconfig

The USB_DWC3_ULPI Kconfig entry uses a UTF-8 non-breaking space (0xca20)
instead of a regular ASCII space (0x20). Commit 2e0d737f ("kconfig:
don't silently ignore unhandled characters") exposes this by warning
about unhandled characters.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>