sgiioc4: sgiioc4_read_status drive busy check fix

Fix the drive non-busy criterion used by sgiioc4_read_status(): neither of the
bits it expects to be set guarantees that the drive is not busy (and might be
interrupting), only the BSY bit itself being zero gurantees that.
While at it, use ATA_BUSY instead of hardcoded value everywhere...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: jeremy@sgi.com
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>