USB: message: cleanup min_t() cast in usb_sg_init()

"length" is type size_t so the cast to unsigned int truncates the
upper bytes.  This isn't an issue in real life (I've checked the
callers) but it's a bit messy.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>