MIPS: Drop unnecessary CONFIG_ISA from RBTX49XX

Those boards do not need CONFIG_ISA if the ne driver could be
selectable without it.  Disable it and update a defconfig.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Jeff Garzik <jeff@garzik.org>