Standardize struct + typedef usage.

[antoinevg]

Depends on #1703

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This PR standardizes struct definitions for HackRF firmware and resolves the ambiguity between declarations such as struct type_t and typedef struct type_t type_t

We prefer two formats:

1. Simple struct definitions:

struct definitions of the form:

struct fpga_driver_t;
typedef struct fpga_driver_t fpga_driver_t;

struct fpga_driver_t {
    ice40_spi_driver_t* bus;
    uint8_t regs[FPGA_NUM_REGS];
    uint8_t regs_dirty;
};

are refactored to:

typedef struct {
	ice40_spi_driver_t* bus;
	uint8_t regs[FPGA_NUM_REGS];
	uint8_t regs_dirty;
} fpga_driver_t;

2. Self-referential struct definitions:

Self-referential definitions of the form:

struct max5864_driver_t;
typedef struct max5864_driver_t max5864_driver_t;

struct max5864_driver_t {
    spi_bus_t* const bus;
    void (*target_init)(max5864_driver_t* const drv);
};

are refactored to:

typedef struct _max5864_driver_t {
    spi_bus_t* const bus;
    void (*target_init)(struct _max5864_driver_t* const drv);
} max5864_driver_t;