network: avoid warning about unaligned pointers

network: avoid warning about unaligned pointers

With gcc-9.0.1-0.10.fc30.x86_64:
../src/network/netdev/macsec.c: In function ‘config_parse_macsec_port’:
../src/network/netdev/macsec.c:584:24: warning: taking address of packed member of ‘struct <anonymous>’ may result in an unaligned pointer value [-Waddress-of-packed-member]
  584 |                 dest = &c->sci.port;
      |                        ^~~~~~~~~~~~
../src/network/netdev/macsec.c:592:24: warning: taking address of packed member of ‘struct <anonymous>’ may result in an unaligned pointer value [-Waddress-of-packed-member]
  592 |                 dest = &b->sci.port;
      |                        ^~~~~~~~~~~~

(The alignment was probably OK, but it's nicer to avoid the warning anyway.)