ncr5380: Reduce max_lun limit

The driver has a limit of eight LUs because of the byte-sized bitfield
that is used for busy flags. That means the maximum LUN is 7. The default
is 8.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Tested-by: Ondrej Zary <linux@rainbow-software.org>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>