nl80211: Fix a copy-paste error in key offload management

nl80211: Fix a copy-paste error in key offload management

Commit b41f26845aaa7cf8aed6e4889e7041debc476ef9 ('Add support for
offloading key management operations to the driver') used incorrect
variable for determining the KCK length. This does not get triggered in
normal use cases since KCK and KEK are always included and of the same
length (in currently supported key management cases). Anyway, this needs
to be fixed to check the correct attribute. (CID 74491)

Signed-off-by: Jouni Malinen <j@w1.fi>