net: slip: fix compilation without NET_L2_ETHERNET

For example it should be possible to compile and use the SLIP module
with NET_L2_DUMMY. This required the following changes:

* Fix a typo in the initializer for struct dummy_api
* Only define eth_capabilities if CONFIG_NET_L2_ETHERNET is defined to
  silence a -Wunused-function compiler warning
* Unconditionally include net/dummy.h

Signed-off-by: Sören Tempel <soeren+git@soeren-tempel.net>