xhci: remove WARN_ON if dma mask is not set for platform devices

The warn on is a bit too much, we will anyway set the dma mask if not set
previously.

The main reason for this fix is that 4.10-rc1  has a dwc3 change that
pass a parent sysdev dev pointer instead of setting the dma mask of
its xhci platform device. xhci platform driver can then get more
attributes from the sysdev than just the dma mask.

The usb core and xhci changes are not yet in 4.10, and a fix like
this was preferred instead of taking those big changes this late in
the rc-cycle.

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