Fix LSO support in TX path

LSO support in the TX path is very broken, which tanks TX throughput.

Before and after fix:
$ iperf -s
------------------------------------------------------------
Server listening on TCP port 5001
TCP window size: 85.3 KByte (default)
------------------------------------------------------------
[  4] local 192.168.2.6 port 5001 connected with 192.168.2.126 port 59379
[ ID] Interval       Transfer     Bandwidth
[  4]  0.0-11.2 sec   432 KBytes   317 Kbits/sec
[  4] local 192.168.2.6 port 5001 connected with 192.168.2.126 port 34801
[  4]  0.0-10.0 sec  18.9 GBytes  16.2 Gbits/sec

Much thanks to Michael Tsymbalyuk for this patch
Remove redundant defines

Remove HW_LSO, DB_LSOMSS(), and ETHERTYPE_VLAN defines--all are
defined in system headers
Add comment to MTU param in vmxnet3s.conf about Sol9/10

This driver only looks at the "MTU" conf parameter on
Solaris 9 and 10, mention that in vmxnet3s.conf