greybus: endo: define endo_init() and endo_exit()

Define init and exit functions to do one-time setup and teardown
of endo-related functionality.  Currently they're place holders;
the next patch will populate them.

Note that we now call gb_operation_exit() from gb_init(), so
we can no longer mark that function with __exit.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>