Revert "serial: core: Use cons->index for preferred console registration"

This reverts commit 91daae03.

The origin patch is causing an issue on r8a7791/koelsch and
r8a7795/salvator-xs platforms where cons->index is not initialized to
expected value.
It is safer to revert this patch for now till it is clear why this is
happening.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Link: https://lore.kernel.org/r/59f51af6bb03fce823663764d17ad0291aa01ab2.1571222199.git.michal.simek@xilinx.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>