mm/m68k: fix build warning of unused variable

Fix build warning of unused variable:

  arch/m68k/mm/init.c: In function 'mem_init': arch/m68k/mm/init.c:151:6: warning: unused variable 'i' [-Wunused-variable]

Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Ungerer <gerg@uclinux.org>
Cc: Thadeu Lima de Souza Cascardo <cascardo@holoscopio.com>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>