xhci: remove GFP_DMA flag from allocation

There is no reason to restrict allocations to the first 16MB ISA DMA
addresses.

It is causing problems in a virtualization setup with enabled IOMMU
(x86_64). The result is that USB is not working in the VM.

CC: <stable@vger.kernel.org>
Signed-off-by: Matthias Lange <matthias.lange@kernkonzept.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>