nubus: Fix pointer validation

Fix bounds checking on slot-space pointer movement.
Remove redundant test for zero byte-lanes value.
Fix broken byte-lanes vs. address validation.

This patch changes the circumstances under which an error is printed to
the console and fixes the address validation.

The validation code should work correctly now: the broken test for a
valid bytelanes value is replaced with a working test (which eliminates
false negatives) and the 24-bit directory offset bounds check is fixed
(which eliminates false positives).

Please see "Designing Cards and Drivers for the Macintosh Family"
ch. 8, "NuBus Card Firmware" for an explanation of the bytelanes check
and directory offset value.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>