RDMA/cxgb4: Configure 0B MRs to match HW implementation

0B MRs need some tweaks to work correctly with HW. When writing the
TPTE, if the MR length is zero we now:

1) turn off all permissions
2) set the length to -1

While functionality/capabilities of the MR are the same with these
changes, it resolves a dapltest 0B RDMA Read test failure.  Based on
original work by Steve Wise <swise@opengridcomputing.com>.

Signed-off-by: Pramod Kumar <pramod@chelsio.com>
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>