fm10k: create "correct" header for the remote end on connect

When we connect to the mailbox, we insert a fake disconnect header so
that the code does not see an invalid header and thus instantly error
every time we bring up the mailbox. However, we incorrectly record the
tail and head from the local perspective. Since the remote end shouldn't
have anything for us, add a "create_fake_disconnect_hdr" function which
inverts the TAIL and HEAD fields. This enables us to connect without any
errors of either TAIL or HEAD incorrectness, and prevents creating
extraneous error messages. This is necessary now since mbx_reset_work
does not actually reset the Tx FIFO head and tail pointers, thus head
and tail might not be equivalent on a reconnect.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>