board/prodrive/p3mx/mv_eth.c: fix GCC 4.6 build warnings

Fix:
mv_eth.c: In function 'mv64460_eth_real_open':
mv_eth.c:471:6: warning: variable 'port_status' set but not used [-Wunused-but-set-variable]
mv_eth.c: In function 'mv64460_eth_stop':
mv_eth.c:722:15: warning: variable 'port_num' set but not used [-Wunused-but-set-variable]
mv_eth.c: In function 'mv64460_eth_xmit':
mv_eth.c:796:15: warning: variable 'port_num' set but not used [-Wunused-but-set-variable]
mv_eth.c: In function 'mv64460_eth_receive':
mv_eth.c:877:15: warning: variable 'port_num' set but not used [-Wunused-but-set-variable]
mv_eth.c: In function 'mv64460_eth_get_stats':
mv_eth.c:979:15: warning: variable 'port_num' set but not used [-Wunused-but-set-variable]
mv_eth.c: In function 'mv64460_eth_update_stat':
mv_eth.c:1006:24: warning: variable 'dummy' set but not used [-Wunused-but-set-variable]
mv_eth.c:1005:15: warning: variable 'port_num' set but not used [-Wunused-but-set-variable]
mv_eth.c: In function 'mv64460_eth_print_stat':
mv_eth.c:1087:15: warning: variable 'port_num' set but not used [-Wunused-but-set-variable]
mv_eth.c: In function 'eth_clear_mib_counters':
mv_eth.c:2141:15: warning: variable 'dummy' set but not used [-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>