hwmon: (nct6775) Rearrange attr-group initialization

We now track the number of attribute groups in nct6775_data, as a
measure to simplify handling differences in the set of enabled
attribute groups between nct6775 drivers (platform & i2c).  As a side
effect, we also reduce the amount of IS_ERR()/PTR_ERR() boilerplate a
bit.

Signed-off-by: Zev Weiss <zev@bewilderbeest.net>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20220427010154.29749-4-zev@bewilderbeest.net
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>