ipg: missing Kconfig dependency

Fix for the error below while linking vmlinux:
[...]
drivers/built-in.o: In function `ipg_ioctl':
drivers/net/ipg.c:2148: undefined reference to `generic_mii_ioctl'
drivers/built-in.o: In function `ipg_get_settings':
drivers/net/ipg.c:2181: undefined reference to `mii_ethtool_gset'
drivers/built-in.o: In function `ipg_set_settings':
drivers/net/ipg.c:2193: undefined reference to `mii_ethtool_sset'
drivers/built-in.o: In function `ipg_nway_reset':
drivers/net/ipg.c:2205: undefined reference to `mii_nway_restart'
make: *** [.tmp_vmlinux1] Error 1

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Avuton Olrich <avuton@gmail.com>
Cc: Jesse Huang <jesse@icplus.com.tw>
Cc: Sorbica Shieh <sorbica@icplus.com.tw>