staging: dwc2: fix value of dma_mask

Passing the value DMA_BIT_MASK(31) to dma_set_mask() causes the
dwc2-pci driver to sometimes fail (cannot enumerate the connected
device). Change it to DMA_BIT_MASK(32) instead, which is a more
sensible value anyway.

Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>