greybus: report right error value

Running "make coccicheck" on the Greybus code discovered that
an error message in gb_camera_debugfs_init() was interpreting
the wrong value in reporting the error code.  Fix that.

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