gianfar: Fix typo bug introduced by move to udp_hdr()

In commit 4bedb452 both the udp and tcp
cases where changed to use udp_hdr() instead of leaving the tcp case
alone and fixing with tcp_hdr().

This ended up causing random behavior with TCP connections because
of looking for tcp_hdr()->check in the wrong place.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>