hwmon: (emc2103) Fix return value

kstrtol() returns appropriate error values. Use those instead of
hardcoding. Silences several sparse messages of following type:
"why not propagate 'result' from kstrtol() instead of (-22)?"

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Steve Glendinning <steve.glendinning@shawell.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>