fbnic: Do not allow mailbox to toggle to ready outside fbnic_mbx_poll_tx_ready

We had originally thought to have the mailbox go to ready in the background
while we were doing other things. One issue with this though is that we
can't disable it by clearing the ready state without also blocking
interrupts or calls to mbx_poll as it will just pop back to life during an
interrupt.

In order to prevent that from happening we can pull the code for toggling
to ready out of the interrupt path and instead place it in the
fbnic_mbx_poll_tx_ready path so that it becomes the only spot where the
Rx/Tx can toggle to the ready state. By doing this we can prevent races
where we disable the DMA and/or free buffers only to have an interrupt fire
and undo what we have done.

Fixes: da3cde08 ("eth: fbnic: Add FW communication mechanism")
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/174654722518.499179.11612865740376848478.stgit@ahduyck-xeon-server.home.arpa


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