net: marvell: prestera: add delayed wq and flush wq on deinit

Flushing workqueues ensures, that no more pending works, related to just
unregistered or deinitialized notifiers. After that we can free memory.

Delayed wq will be used for neighbours in next patches.

Co-developed-by: Taras Chornyi <tchornyi@marvell.com>
Signed-off-by: Taras Chornyi <tchornyi@marvell.com>
Co-developed-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Signed-off-by: Oleksandr Mazur <oleksandr.mazur@plvision.eu>
Signed-off-by: Yevhen Orlov <yevhen.orlov@plvision.eu>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>