small cleanup in ax25_addr_parse()

The comments were wrong here because "AX25_MAX_DIGIS" is 8 but the
comments say 6.  Also I've changed the "7" to "AX25_ADDR_LEN".

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>