Low-level module handle structure

The SPIDEV structure is the handle that the low-level module has to return to the high-level code. You can extend the structure, but SPIDEV has to be at the top. This handle is also passed to the low-level driver when the high-level code calls low-level functions.

typedef struct _spidev_entry {
    iofunc_attr_t   attr;
    void        *hdl;       /* Pointer to high-level handle */
    void        *lock;      /* Pointer to lock list */