ixgb: fix TX hangs under heavy load

A merge error occurred where we merged the wrong block here
in version 1.0.120. The right condition for frags is slightly
different then for the skb, so account for the difference properly
and trim the TSO based size right.

Originally part of a fix reported by IBM to fix TSO hangs on
pSeries hardware.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Cc: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Jeff Garzik <jeff@garzik.org>