hwmon: (mcp3021) use permission-specific DEVICE_ATTR variants

hwmon: (mcp3021) use permission-specific DEVICE_ATTR variants

Use DEVICE_ATTR_RO for read-only attributes. This simplifies the source
code, improves readbility, and reduces the chance of inconsistencies.

The conversion was done automatically using coccinelle. It was validated
by compiling both the old and the new source code and comparing its text,
data, and bss size.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
[groeck: Updated description]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>