greybus: endo: Fix compilation warning

WARNING: /home/viresh/work/repos/ara/greybus/greybus.o(.init.text+0xb8):
Section mismatch in reference from the function init_module() to the
function .exit.text:gb_endo_exit()

The function __init init_module() references
a function __exit gb_endo_exit().

Fix it by removing __exit from endo_exit().

Fixes: cf64356c5151 ("endo: define endo_init() and endo_exit()")
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>