macsec: Fix header length if SCI is added if explicitly disabled

Even if sending SCIs is explicitly disabled, the code that creates the
Security Tag might still decide to add it (e.g. if multiple RX SCs are
defined on the MACsec interface).
But because the header length so far only depended on the configuration
option the SCI overwrote the original frame's contents (EtherType and
e.g. the beginning of the IP header) and if encrypted did not visibly
end up in the packet, while the SC flag in the TCI field of the Security
Tag was still set, resulting in invalid MACsec frames.

Fixes: c09440f7 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Tobias Brunner <tobias@strongswan.org>
Acked-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>