qla3xxx: cleanup checksum offload code

The code for checksum is more complex than needed when dealing with VLAN's;
the higher layers already pass down the location of the IP header.

Compile tested only, no hardware available.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Ron Mercer <ron.mercer@qlogic.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>