hwmon: (pmbus/mp2856) Let enum chips start with index 0

hwmon: (pmbus/mp2856) Let enum chips start with index 0

Earlier it was assumed that the data pointer in of_device_id must not start
with 0 (NULL) if i2c_get_match_data() is used. However, it turns out that
this is perfectly fine as long as there is also an i2c_device_id array with
the same data, which is used as fallback in that case.

Let enum chips start with 0 to avoid confusion against other drivers
where the enum starts with 0 and i2c_get_match_data() is used as well.

While doing that, remove chip_id from struct mp2856_data since it is only
used in the probe function, and typecast the result of i2c_get_match_data()
to kernel_ulong_t to avoid the double typecast.

Cc: Peter Yin <peteryin.openbmc@gmail.com>
Cc: Potin Lai <potin.lai.pt@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>