Commit 3c5317e9046 ("usb: xhci-hcd: Make use of lo_hi_readq/writeq()")
had made use of lo_hi_readq/writeq() to implement xhci_read/write_64(),
like Linux does, but this was removed in commit fddd1c7c51a8
("usb: host: remove xhci driver"). Reinstate it to make code clearer.

No functional change.

[Note: Linux doesn't use any readq/writeq, but does two 32-bit access
 always, even on 64-bit systems]

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>