msk: Enable MSI
igb: Fix comment; 82575 expects TX context index instead of ring index
emx: Update comment about multiple TX queues on 82574

The watchdog timeout on TX queue0 when multiple TX queues are enable
is _not_ related to hardware TX checksum offloading.

In the latest 82574 specification (Feb 2013, rev 3.3), it claims the
same TX context setting up requirement for multiple TX queues as with
82571.  However, 82571 works quite well with multiple TX queues.

Sigh.