ide-mpc8xx: make m8xx_ide_init_ports() return an error value

* Make m8xx_ide_init_ports() return an error value.

* Update mpc8xx_ide_probe() to setup/probe only ports for which
  m8xx_ide_init_ports() succedded.

Cc: Vitaly Bordug <vitb@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>