usb: xhci: Add missing cache flush in the scratchpad array initialization

In current code there is no cache flush after initializing the scratchpad
buffer array with the scratchpad buffer pointers. This leads to a failure
of the "slot enable" command on the rpi4 board (Broadcom STB PCIe
controller + VL805 USB hub) - the very first TRB transfer on the command
ring fails and there is a timeout while waiting for the command completion
event. After adding the missing cache flush everything seems to be working
as expected.

Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>