[AX.25]: Fix unchecked ax25_protocol_register uses.

Replace ax25_protocol_register by ax25_register_pid which assumes the
caller has done the memory allocation.  This allows replacing the
kmalloc allocations entirely by static allocations.

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