staging: dgap: fix smatch warn: unsigned var is never less than zero

This patch fixes a smatch warning about an unsigned
integer being tested for less than zero.

Signed-off-by: Mark Hounschell <markh@compro.net>
Tested-by: Mark Hounschell <markh@compro.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>