net: stmmac: ensure jumbo_frm error return is correctly checked for -ve value

The current comparison of entry < 0 will never be true since entry is an
unsigned integer. Make entry an int to ensure -ve error return values
from the call to jumbo_frm are correctly being caught.

Detected by CoverityScan, CID#1238760 ("Macro compares unsigned to 0")

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