net: stmmac: fix a broken u32 less than zero check

The check that queue is less or equal to zero is always true
because queue is a u32; queue is decremented and will wrap around
and never go -ve. Fix this by making queue an int.

Detected by CoverityScan, CID#1428988 ("Unsigned compared against 0")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>