[PATCH] ixgb: fix tx unit hang - properly calculate desciptor count

There were some tso bugs that only showed up with heavy load and 16kB
pages that this patch fixes by making the driver's internal use count
of descriptors match the count that it was estimating it needed using
the DESC_NEEDED macro.  This bug caused NETDEV_WATCHDOG resets aka
tx timeouts.

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