[AX.25]: Rename ax25_encapsulate to ax25_hard_header

Rename ax25_encapsulate to ax25_hard_header which these days more
accurately describes what the function is supposed to do.

Signed-off-by: Ralf Baechle DL5RB <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>