ixgbevf: fix handling of tx checksumming

This patch resolves an issue introduced by:
commit 7ad1a093
ixgbevf: make the first tx_buffer a repository for most of the skb info

Incorrect check for the result of ixgbevf_tso() can lead to calling
ixgbevf_tx_csum() which can spawn 2 context descriptors and result in
performance degradation and/or corrupted packets.

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>