hwmon: (adm9240) Store i2c device instead of client in local data

We only use the pointer to i2c_client to access &client->dev.
Store the device pointer directly instead of retrieving it
from i2c_client.

Cc: Chris Packham <Chris.Packham@alliedtelesis.co.nz>
Reviewed-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Tested-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>