usb: host: xhci-plat: fix NULL pointer in probe for device tree case

During probe, in the device tree case, the data pointer associated to a
compatible is dereferenced. However, not all the compatibles are
associated to a private data pointer.

The generic-xhci and the xhci-platform don't need them, this patch adds a
test on the data pointer before accessing it, avoiding a kernel crash.

Fixes: 4efb2f69 ("usb: host: xhci-plat: add struct xhci_plat_priv")
Cc: stable@vger.kernel.org
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>