hwmon: (adt7462) Fix an error return in ADT7462_REG_VOLT()

This is only called from adt7462_update_device().  The caller expects it
to return zero on error.  I fixed a similar issue earlier in commit
a4bf06d5 ("hwmon: (adt7462) ADT7462_REG_VOLT_MAX() should return 0")
but I missed this one.

Fixes: c0b4e3ab ("adt7462: new hwmon driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Link: https://lore.kernel.org/r/20200303101608.kqjwfcazu2ylhi2a@kili.mountain


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