net: macsec: remove the prepare phase when offloading

The hardware offloading in MACsec was initially supported using 2 phases.
This was proposed in the RFC as this could have allowed easier fallback
to the software implementation if the hardware did not support a feature
or had enough entries already. But this fallback wasn't implemented and
might not be a good idea after all. In addition it turned out this logic
didn't mapped well the hardware logic and device drivers were mostly
ignoring the preparation phase.

Let's remove this as it does not offer any advantage and is ignored by
drivers.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>