nubus: Avoid array underflow and overflow

Check array indices. Avoid sprintf. Use buffers of sufficient size.
Use appropriate types for array length parameters.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>