RDMA/cxgb4: Initialize the device status page

The status page is mapped to user processes and allows sharing the
device state between the kernel and user processes.  This state isn't
getting initialized and thus intermittently causes problems.  Namely,
the user process can mistakenly think the user doorbell writes are
disabled which causes SQ work requests to never get fetched by HW.

Fixes: 05eb2389 ("cxgb4/iw_cxgb4: Doorbell Drop Avoidance Bug Fixes").
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Cc: <stable@vger.kernel.org> # v3.15
Signed-off-by: Roland Dreier <roland@purestorage.com>