xhci: fix platform quirks overwrite regression in 4.7-rc1

commit b1c127ae ("usb: host: xhci: plat: make use of new methods in
xhci_plat_priv") sets xhci->quirks before calling xhci_gen_setup(), which
will overwrite them.

Don't overwite the quirks, just add the new ones

Fixes: b1c127ae ("usb: host: xhci: plat: make use of new methods in xhci_plat_priv")
Reported-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>