hwmon: (mcp3021) Convert to use devm_ functions

Convert to use devm_ functions to reduce code size and simplify the code.

Cc: Mingkai Hu <Mingkai.hu@freescale.com>
Cc: Xie Xiaobo <X.Xie@freescale.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>