arm: rpi: perform XHCI firmware upload only once

XHCI firmware upload must be performed only once after initializing the
PCI bridge. This fixes USB stack initialization after calling "usb stop;
usb start" on Raspberry Pi 4B.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Signed-off-by: Matthias Brugger <mbrugger@suse.com>