IB/ehca: Serialize hypervisor calls in ehca_register_mr()

Some pSeries hypervisor versions show a race condition in the allocate
MR hCall.  Serialize this call per adapter to circumvent this problem.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>