hwmon: (asc7621) remove redundant assignment to newval

The setting of newval to zero is redundant as the following if/else
stanzas will always update newval to a new value. Remove the
redundant setting, cleans up clang build warning:

drivers/hwmon/asc7621.c:582:2: warning: Value stored to 'newval' is
never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>