m68k: modify cache push and clear code for ColdFire with MMU enable

The cache push and clear code only need to flush the branch cache on
the write-through cache setup of the ColdFire V4e with MMU enabled.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Matt Waddel <mwaddel@yahoo.com>
Acked-by: Kurt Mahan <kmahan@xmission.com>