Prevent attempt to write write_cmos_register() if read_cmos_register() failed

[maksqwe]

uint16_t reg = read_cmos_register(dev, 0x0106);
if (reg < 0)

'reg' never be < 0. If read_cmos_register() failed it return value < 0, reg = 0xffff - err_value + 1

In read_cmos_register() send_cmd() if failed can return -1 or LIBUSB_ERROR_OTHER(-99) therefore read_cmos_register() must return only -1 if failed for correct result check

[piedar]

This is definitely a bug, but I don't think your solution works either because it compares a uint16_t with -1, which will never be true:

#include <stdint.h>
#include <stdio.h>

int main()
{
    uint16_t foo = -1;
    if (foo == -1)
        printf("Never true!\n");

    return 0;
}

I made read_register() and read_cmos_register() return UINT16_MAX on error. It's not perfect, but it's better than the alternatives:

1. Pass in an int* error parameter. (yuck)
2. Return an int32_t and require a cast to get the actual value. (double-yuck)