macsec: allow CKN shorter than 64 characters
See wpa_supplicant commit [1]:
macsec: Make pre-shared CKN variable length
IEEE Std 802.1X-2010, 9.3.1 defines following restrictions for
CKN:
"MKA places no restriction on the format of the CKN, save that it
comprise an integral number of octets, between 1 and 32
(inclusive), and that all potential members of the CA use the same
CKN. No further constraints are placed on the CKNs used with PSKs,
..."
Hence do not require a 32 octet long CKN but instead allow a
shorter CKN to be configured.
This fixes interoperability with some Aruba switches, that do not
accept a 32 octet long CKN (only support shorter ones).
[1] https://w1.fi/cgit/hostap/commit/?id=b678ed1efc50e8da4638d962f8eac13312a4048f