hwmon: (tmp421) support HWMON_T_ENABLE

Since the recent patches added possibility of disabling sensor channels
via DT, it only make sense to allow controlling that from userspace via
HWMON_T_ENABLE mechanism. This patches adds support for that.

Signed-off-by: Krzysztof Adamski <krzysztof.adamski@nokia.com>
Link: https://lore.kernel.org/r/a64c22e7323bd5a083f37aaaca91a745ac1beef3.1634206677.git.krzysztof.adamski@nokia.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>