staging: r8188eu: clean up on_action_public

staging: r8188eu: clean up on_action_public

Use the struct mgmt to read the action_code. This is much simpler than
parsing the message ourselves.

Add a comment about reading the action code. All members of the action
enum start with an action_code byte. It does not matter which member we
use.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20221030173326.1588647-10-martin@kaiser.cx
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>