Serialize struct am7xxx_header properly before sending it on the wire

That is in order to:

  1. keep data in the structs always in the host native byte order, this
     is more natural and less error prone as we might forget to use
     htole32() when setting struct fields or le32toh() when accessing
     them;

  2. be more portable: this way the buffer sent to the wire is
     independent of struct alignments or paddings introduced by
     compilers or required by a particular architecture.