RDMA/qedr: Fix wmb usage in qedr

This patch comes as a result of Sinan Kaya's work and the decision that
writel() must be a strong enough barrier for DMA.

wmb usages in qedr driver have either been removed where they were there
only to order DMA accesses, and replaced with smp_wmb and comments for the
places that the barrier was there for SMP reasons.

Fixes: 561e5d48 ("RDMA/qedr: eliminate duplicate barriers on weakly-ordered archs")
Signed-off-by: Michal Kalderon <Michal.Kalderon@cavium.com>
Signed-off-by: Ariel Elior <Ariel.Elior@cavium.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>