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

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