eth: fbnic: Add support for multiple concurrent completion messages

Extend fbnic mailbox to support multiple concurrent completion messages at
once. This enables fbnic to support running multiple operations at once
which depend on a response from firmware via the mailbox.

Signed-off-by: Lee Trager <lee@trager.us>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250512190109.2475614-4-lee@trager.us


Signed-off-by: Paolo Abeni <pabeni@redhat.com>