ethernet: select CONFIG_CRC32 as needed

A number of ethernet drivers require crc32 functionality to be
avaialable in the kernel, causing a link error otherwise:

arm-linux-gnueabi-ld: drivers/net/ethernet/agere/et131x.o: in function `et1310_setup_device_for_multicast':
et131x.c:(.text+0x5918): undefined reference to `crc32_le'
arm-linux-gnueabi-ld: drivers/net/ethernet/cadence/macb_main.o: in function `macb_start_xmit':
macb_main.c:(.text+0x4b88): undefined reference to `crc32_le'
arm-linux-gnueabi-ld: drivers/net/ethernet/faraday/ftgmac100.o: in function `ftgmac100_set_rx_mode':
ftgmac100.c:(.text+0x2b38): undefined reference to `crc32_le'
arm-linux-gnueabi-ld: drivers/net/ethernet/freescale/fec_main.o: in function `set_multicast_list':
fec_main.c:(.text+0x6120): undefined reference to `crc32_le'
arm-linux-gnueabi-ld: drivers/net/ethernet/freescale/fman/fman_dtsec.o: in function `dtsec_add_hash_mac_address':
fman_dtsec.c:(.text+0x830): undefined reference to `crc32_le'
arm-linux-gnueabi-ld: drivers/net/e...