fm10k/base: fix mailbox connect

fm10k/base: fix mailbox connect

When we connect to the mailbox, we insert a fake disconnect header so
that the code does not see an error 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 clear the Tx FIFO head and tail pointers.

Signed-off-by: Wang Xiao W <xiao.w.wang@intel.com>