RDMA/ocrdma: Fail connection for MTU lesser than 512

HW currently restricts the IB MTU range between 512 and 4096.
Fail connection for MTUs lesser than 512.

Signed-off-by: Naga Irrinki <naga.irrinki@avagotech.com>
Signed-off-by: Selvin Xavier <selvin.xavier@avagotech.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>