net: macsec: Add endianness annotations in salt struct

This change resolves warning produced by sparse tool as currently
there is a mismatch between normal generic type in salt and endian
annotated type in macsec driver code. Endian annotated types should
be used here.

Sparse output:
warning: restricted ssci_t degrades to integer
warning: incorrect type in assignment (different base types)
    expected restricted ssci_t [usertype] ssci
    got unsigned int
warning: restricted __be64 degrades to integer
warning: incorrect type in assignment (different base types)
    expected restricted __be64 [usertype] pn
    got unsigned long long

Signed-off-by: Ales Nezbeda <anezbeda@redhat.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Signed-off-by: David S. Miller <davem@davemloft.net>