Add port_xmit_wait to the error counters read by mlx5_ib_process_mad to
ensure sysfs port counter provides correct value for PortXmitWait.
Otherwise the sysfs port_xmit_wait file always contains zero.

The previous MAD_IFC implementation populated this counter, but it was
removed during the migration to PPCNT for error counters (32-bit only).
Signed-off-by: NTim Wright <tim@binbash.co.uk>
Signed-off-by: NDoug Ledford <dledford@redhat.com>