bnx2x: change FW GRO error message to WARN_ONCE

It's supposed to be impossible for TPA to give us anything else
than IPv4 or IPv6 here. But in case there is a way to reach this error
by some strange received frames, we don't want to flood the kernel log.
WARN_ONCE is better for this purpose.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>