usb: xhci: Add missing xhci_readl()

Accessing the xHCI controller registers should be done via the
xhci_readl/writel functions. This patch adds this to a few missing
places.

Signed-off-by: Aaron Williams <awilliams@marvell.com>
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Chandrakala Chavva <cchavva@marvell.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Marek Vasut <marex@denx.de>