hwmon: (emc2103) Fix use of an uninitilized variable in error case

Fix:

emc2103.c: In function set_pwm_enable:
emc2103.c:463:12: warning: conf_reg may be used uninitialized in this function

by checking the return value from read_u8_from_i2c(). This fixes a real problem,
as conf_reg is really uninitialized if read_u8_from_i2c returns an error.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>
Acked-by: Jean Delvare <khali@linux-fr.org>