net: mvpp2: cls: fix less than zero check on a u32 variable

The signed return from the call to mvpp2_cls_c2_port_flow_index is being
assigned to the u32 variable c2.index and then checked for a negative
error condition which is always going to be false. Fix this by assigning
the return to the int variable index and checking this instead.

Addresses-Coverity: ("Unsigned compared against 0")
Fixes: 90b509b3 ("net: mvpp2: cls: Add Classification offload support")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>