xhci: Don't require a valid get_quirks() function pointer during xhci setup

Not all platforms drivers need to set up custom quirks during the xhci
generic setup. Allow them to pass NULL as the function pointer when
calling xhci_gen_setup()

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Message-ID: <20230602144009.1225632-4-mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>