usb, ohci: fix ohci swap register access

commit 57faca19 ("drivers: USB: OHCI: allow compilation for 64-bit targets")
broke ohci support for the mpc85xx based socrates board,
as it removed volatile keyword from ohci_readl/writel.

Fix this so usb works again on socrates board.

Signed-off-by: Heiko Schocher <hs@denx.de>