hwmon: (emc6w201) Do not declare enum variable

Fix the following smatch warning:

emc6w201.c:52:26: warning: symbol 'subfeature' was not declared.
Should it be static?

'enum { } subtype' declares an enum as (global) variable which we don't want.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jean Delvare <khali@linux-fr.org>