net: phy: mscc: fix macsec key length

The key length used to store the macsec key was set to MACSEC_KEYID_LEN
(16), which is an issue as:
- This was never meant to be the key length.
- The key length can be > 16.

Fix this by using MACSEC_MAX_KEY_LEN instead (the max length accepted in
uAPI).

Fixes: 28c5107a

 ("net: phy: mscc: macsec support")
Reported-by: Lior Nahmanson <liorna@nvidia.com>
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>