4xx_enet.c: Fix GCC 4.6 build warnings

Fix:
4xx_enet.c: In function 'ppc_4xx_eth_init':
4xx_enet.c:875:6: warning: variable 'ethgroup' set but not used [-Wunused-but-set-variable]

I used "__maybe_unused" here intentionally, since all other
alternatives to fix this compilation warning would result in more
ifdef's.

Signed-off-by: Stefan Roese <sr@denx.de>