IB core: Add port_xmit_wait counter

IB core: Add port_xmit_wait counter

Add the missing port_xmit_wait counter. This counter is displayed through
some tools like perfquery but is not available via sysfs.

For the PORT_PMA_ATTR macro the _counter field is set to zero
allowing us to specify the offset directly like with PORT_PMA_ATTR_EXT

See also the earlier work in 2008 by Vladimir Skolovsky

https://www.mail-archive.com/general@lists.openfabrics.org/msg20313.html

Signed-off-by: Vladimir Sokolvsky <vlad@mellanox.com>
Signed-off-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>