usb: xhci: Add missing endian conversions (cpu_to_leXX / leXX_to_cpu)

While trying to use the U-Boot xHCI driver on the MIPS Octeon platform,
which is big endian, I noticed that the driver is missing a few endian
conversion calls. This patch adds these missing endian conversion
calls.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Marek Vasut <marex@denx.de>