hwmon: (max6697) Allow max6581 to create tempX_offset

hwmon: (max6697) Allow max6581 to create tempX_offset

Add tempX_offset attribute support to max6697 driver.
Temperature offsets are only supported for MAX6581.

Testing:
echo 16250 > temp2_offset
cat temp2_offset
16250

echo 17500 > temp3_offset
cat temp3_offset
17500
cat temp4_offset
0
cat temp2_offset
17500

echo 0 > temp2_offset
cat temp2_offset
0
cat temp3_offset
17500

echo -0 > temp2_offset
cat temp2_offset
0

echo -100000 > temp2_offset
cat temp2_input
4875

echo 10000 > temp2_offset
cat temp2_input
47125

echo -2000 > temp2_offset
cat temp2_input
34875

echo -0 > temp2_offset
cat temp2_input
37000

Signed-off-by: Chu Lin <linchuyuan@google.com>
Link: https://lore.kernel.org/r/20200707171528.2929275-1-linchuyuan@google.com
[groeck: Fixed checkpatch warning (unnecessary { })]
Signed-off-by: Guenter Roeck <linux@roeck-us.net>