drivers: ethernet: add dependency to CONFIG_ETH_INIT_PRIORITY

Add a dependency on CONFIG_NET_L2_ETHERNET for
CONFIG_ETH_INIT_PRIORITY. This hides the Ethernet driver initialization
priority option in menuconfig unless CONFIG_NET_L2_ETHERNET is selected.

CONFIG_ETH_INIT_PRIORITY cannot currently be moved inside the
CONFIG_ETH_DRIVER if-statement in Kconfig as it is used outside the
Ethernet driver source files.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>