xhci: Clean up 32-bit build warnings.

Randy Dunlap points out that commit 9258c0b2 "xhci: Better debugging for
critical host errors." introduces some new build warnings on 32-bit
builds:

drivers/usb/host/xhci-ring.c:1936:3: warning: format '%016llx' expects type 'long long unsigned int', but argument 3 has type 'dma_addr_t'
drivers/usb/host/xhci-ring.c:1958:3: warning: format '%016llx' expects type 'long long unsigned int', but argument 3 has type 'dma_addr_t'

Cast the results of xhci_trb_virt_to_dma() from a dma_addr_t to an
unsigned long long.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Reported-by: Randy Dunlap <rdunlap@xenotime.net>