usb: xhci: xhci_mem_init: Use cpu_to_le64() and not xhci_writeq()

xhci_writeq() makes the CPU->LE swapping only when addressing registers
in the xHCI controller address range and not in the local memory (RAM).
We need to use cpu_to_le64() here to ensure that the conversion is done
correctly.

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>