RDMA/cxgb4: Fix endianness of addition to mpa->private_data_size

sparse correctly warns that if mpa->private_data_size is __be16, then
doing += on it is wrong, even if we do += htons(<something>) -- on a
little endian system, carries will go the wrong way.  Fix this up by
doing the addition in native byte order.

Acked-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>