ixgbe: properly return CHECKSUM_NONE, cleanup csum code

We were not returning CHECKSUM_NONE in a lot of cases which is
wrong. Move common exit points in this function and error code
up before the actual work in this function.

Signed-off-by: Ayyappan Veeraiyan <ayyappan.veeraiyan@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>