greybus: pm: add error handling to bundle activation

The firmware now keeps the underlying hardware disabled until
receiving the first Bundle Activate request. Additionally: requesting
transition to a state the bundle is already in is no longer an error.

We can now add proper error handling to the bundle activate call.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>