other initializations succeeded.
- Initialize the TX and RX rings in epic_attach() rather than in
epic_init() where we're not supposed to fail. Similarly, free
the TX and RX rings in epic_detach() rather than in epic_stop().
- Change epic_init() to be a void function now that it can't fail.
Also change its parameter to a void * so that we have a correct
prototype for if_init.
- Now that epic_init() has a correct prototype, don't cast the
function pointer when initializing if_init.
- Fix nearby style bugs.