[PATCH] IB/mthca: Move WQE structures into their own header

Move the definitions of the WQE structures from mthca_qp.c into
mthca_wqe.h, so that we'll be able to share them when we add the
SRQ code in mthca_srq.c.

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