hwmon: (ltc2991) re-order conditions to fix off by one bug

In the Linux kernel, the following vulnerability has been resolved:

hwmon: (ltc2991) re-order conditions to fix off by one bug

LTC2991_T_INT_CH_NR is 4. The st->temp_en[] array has LTC2991_MAX_CHANNEL (4) elements. Thus if "channel" is equal to LTC2991_T_INT_CH_NR then we have read one element beyond the end of the array. Flip the conditions around so that we check if "channel" is valid before using it as an array index.