m68k: amiga: Fix Denise detection on OCS

The "default" statement for detecting an original Denise chip seems to
be misplaced, causing original Denise chips not being detected.

Fix this by moving it from the outer to the inner "switch" statement.

Fortunately no code relies on this, but the detected version is printed
during boot, and available through /proc/hardware.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20200112171705.22600-1-geert@linux-m68k.org