net: wan: wanxl: define CROSS_COMPILE_M68K

It was used but never set.  The hardcoded value from before the dawn of
time was non-standard; the usual name for cross-tools is $TRIPLET-$TOOL

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Signed-off-by: David S. Miller <davem@davemloft.net>