cpmac: remove hopeless #warning

The #warning was present 10 years ago when the driver first got merged.
As the platform is rather obsolete by now, it seems very unlikely that
the warning will cause anyone to fix the code properly.

kernelci.org reports the warning for every build in the meantime, so
I think it's better to just turn it into a code comment to reduce
noise.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>