mka: Fix getting capabilities from the driver

mka: Fix getting capabilities from the driver

In commit a25e4efc9e428d968e83398bd8c9c94698ba5851 ('mka: Add driver op
to get macsec capabilities') I added some code to check the driver's
capabilities. This commit has two problems:
 - wrong enum type set in kay->macsec_confidentiality
 - ignores that drivers could report MACSEC_CAP_NOT_IMPLEMENTED, in
   which case the MKA would claim that MACsec is supported.

Fix this by interpreting MACSEC_CAP_NOT_IMPLEMENTED in the same way as a
DO_NOT_SECURE policy, and set the correct value in
kay->macsec_confidentiality.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>