net: macsec: move sci_to_cpu to macsec header

Move sci_to_cpu to the MACsec header to use it in drivers.

Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>