IB/ipath: Add capability to modify PBC word

During compliance testing and when debugging some interconnect issues,
it is very useful to be able to send malformed packets, without having
the device signal them as malformed (drop, or terminate with EBP). The
hardware supports this, but the driver "diagnostic packet" interface
did not.

Extend capability to send specific malformed packets for testing.

Signed-off-by: Michael Albaugh <Michael.Albaugh@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>