vmxnet3: Fix TSO support on VMware Player/Workstation/Fusion

There's a report that TSO support on VMware Player/Workstation/Fusion was broken on upstream OS of this driver(FreeBSD):
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=185849
This patch is backported from the fix on FreeBSD:
http://svnweb.freebsd.org/base?view=revision&revision=267253

fixed #340

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>