m68k: fix ColdFire node shift size calculation

The m68k pg_data_table is a fix size array defined in arch/m68k/mm/init.c.
Index numbers within it are defined based on memory size. But for Coldfire
these don't take into account a non-zero physical RAM base address, and this
causes us to access past the end of this array at system start time.

Change the node shift calculation so that we keep the index inside its range.

Reported-by: Angelo Dureghello <angelo@sysam.it>
Tested-by: Angelo Dureghello <angelo@sysam.it>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>