usb/bus: Remove dead assignment in usb_get_fw_dev_path()

usb/bus: Remove dead assignment in usb_get_fw_dev_path()

Clang static code analyzer show warning:
qemu/hw/usb/bus.c:615:13: warning: Value stored to 'pos' is never read
            pos += snprintf(fw_path + pos, fw_len - pos, "%s@%lx",

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20200827110311.164316-9-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>