usb: xhci: allow multiple firmware versions

Allow multiple firmware file versions in table and load them in
increasing order as we find them in the file system.

Signed-off-by: Vinod Koul <vkoul@kernel.org>
Cc: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Cc: Christian Lamparter <chunkeey@googlemail.com>