nubus: Fix log spam

Testing shows that a single Radius PrecisionColor 24X display board,
which has 95 functional resources, produces over a thousand lines of
log messages. Suppress these messages with pr_debug().
Remove some redundant messages relating to nubus_get_subdir() calls.
Fix the format block debug messages as the sequence of entries is
backwards (my bad).
Move the "scanning slots" message to its proper location.

Fixes: 71ae40e4 ("nubus: Clean up printk calls")
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>