hwmon: (pmbus/max31827) Explain why enum chips must not start with 0

If a driver calls device_get_match_data(), the .data pointer in its id
data structures must not be NULL/0 because device_get_match_data()
returns NULL if an entry is not found. Explain that in a comment to avoid
confusion why this is required in this driver but not in other drivers.

Cc: Daniel Matyas <[e-mail]>
Acked-by: Nuno Sa <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>

drivers/hwmon/max31827.c

···
 #define MAX31827_M_DGR_TO_16_BIT(x)	(((x) << 4) / 1000)
 #define MAX31827_DEVICE_ENABLE(x)	((x) ? 0xA : 0x0)
 
+/*
+ * The enum passed in the .data pointer of struct of_device_id must
+ * start with a value != 0 since that is a requirement for using
+ * device_get_match_data().
+ */
 enum chips { max31827 = 1, max31828, max31829 };
 
 enum max31827_cnv {