fbnic: Gate AXI read/write enabling on FW mailbox

In order to prevent the device from throwing spurious writes and/or reads
at us we need to gate the AXI fabric interface to the PCIe until such time
as we know the FW is in a known good state.

To accomplish this we use the mailbox as a mechanism for us to recognize
that the FW has acknowledged our presence and is no longer sending any
stale message data to us.

We start in fbnic_mbx_init by calling fbnic_mbx_reset_desc_ring function,
disabling the DMA in both directions, and then invalidating all the
descriptors in each ring.

We then poll the mailbox in fbnic_mbx_poll_tx_ready and when the interrupt
is set by the FW we pick it up and mark the mailboxes as ready, while also
enabling the DMA.

Once we have completed all the transactions and need to shut down we call
into fbnic_mbx_clean which will in turn call fbnic_mbx_reset_desc_ring for
each ring and shut down the DMA and once again invalidate the descriptors.

Fixes: 36461531 ("eth: fbnic: Add register init to set PCIe/Ethernet device config")
Fixes: da3cde08 ("eth: fbnic: Add FW communication mechanism")
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/174654718623.499179.7445197308109347982.stgit@ahduyck-xeon-server.home.arpa


Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>