greybus: es2: Fix apb_log null pointer exception

Read on apb_log causes null pointer exception due to the missing es2
device pointer passing to the debugfs.

Testing done:
- Enable apb_log and cat it

Signed-off-by: David Lin <dtwlin@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>