m68k/amiga: Fix "debug=mem"

`debug=mem' on Amiga has been broken for a while.
early_param() processing is done very/too early, i.e. before
amiga_identify() / amiga_chip_init(), causing amiga_savekmsg_setup() not
to find any Chip RAM.

As we don't plan to free this memory anyway, just steal it from the initial
Chip RAM memory block instead.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>