RDMA/cma: Avoid assigning an IS_ERR value to cm_id pointer in CMA id object

Avoid assigning an IS_ERR value to the cm_id pointer.  This fixes a
few anomalies in the error flow due to confusion about checking for
NULL vs IS_ERR, and eliminates the need to test for the IS_ERR value
every time we wish to determine if the cma_id object has a cm device
associated with it.

Also, eliminate the now-unnecessary procedure cma_has_cm_dev (we can
check directly for the existence of the device pointer -- for a
non-NULL check, makes no difference if it is the iwarp or the ib
pointer).

Finally, make a few code changes here to improve coding consistency.

Signed-off-by: Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: Roland Dreier <roland@purestorage.com>