enic(4): fix down/up, MTU changes and more

ifconfig down/up cycles was not working. Fix that which is required
to support MTU changes. Now doing ifconfig enic0 mtu 3000 for example
works. If the MTU is changes in the VIC HW configuration, that is not
reflected in and the OS reports the default 1500. I need to look at
that but changing it via ifconfig works. So this is different then
what Linux does.

Change TX interrupt allocation to be in this driver. Change the admin
interrupt count to 2. This make multiple queues work but need to be
done as pairs so if the VIC has more TX or RX queues setup in the
VIC configuration it will use the lesser value.

While updating the TX interrupt also add support for devcmd2.

Enable checksum offloading.

PR: 282095