[netdrvr] ne: Use CONFIG_MACH_TX49XX

After some cleanups in arch/mips area, now MACH_TX49XX is selected for
both TOSHIBA_RBTX4927 and TOSHIBA_RBTX4938.  Fold these two conditions
to one.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>