net: Revert ARCNET and PHYLIB to tristate options

commit 88491d8103498a6166f70d5999902fec70924314 ("drivers/net: Kconfig
& Makefile cleanup") changed the type of these options to bool, but
they select code that could (and still can) be built as modules.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>