hwmon: (core) Clarify when read and write callbacks are mandatory

The callback descrption in hwmon.h was misleading and stated that read and
write callbacks would be optional. More accurate is is that the callbacks
are mandatory if readable / writeable attributes are present.
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

include/linux/hwmon.h

@@ -298,8 +298,7 @@ enum hwmon_pwm_attributes {
  *			Channel number
  *		The function returns the file permissions.
  *		If the return value is 0, no attribute will be created.
- * @read:       Read callback. Optional. If not provided, attributes
- *		will not be readable.
+ * @read:	Read callback. Mandatory if readable attributes are present.
  *		Parameters are:
  *		@dev:	Pointer to hardware monitoring device
  *		@type:	Sensor type
@@ -308,8 +307,7 @@ enum hwmon_pwm_attributes {
  *			Channel number
  *		@val:	Pointer to returned value
  *		The function returns 0 on success or a negative error number.
- * @write:	Write callback. Optional. If not provided, attributes
- *		will not be writable.
+ * @write:	Write callback. Mandatory if writeable attributes are present.
  *		Parameters are:
  *		@dev:	Pointer to hardware monitoring device
  *		@type:	Sensor type