mka: Support for 256-bit SAK generation

mka: Support for 256-bit SAK generation

There is already partial support of GCM-AES-256. It is possible to
enable this mode by setting 'kay->macsec_csindex = 1;' in
ieee802_1x_kay_init() function, but the generated key contained only 128
bits of data while other 128 bits are in 0.

Enables KaY to generate full 256-bit SAK from the same 128-bit CAK. Note
that this does not support 256-bit CAK or AES-CMAC-256 -based KDF.

Signed-off-by: Andrey Kartashev <andrey.kartashev@afconsult.com>