hwmon: (twl4030-madc-hwmon) Fix warning message caused by removal of __devexit

Commit 281dfd0 removed __devexit from the exit function but left __exit_p in
place. This results in the following warning message if the module is built into
the kernel.

twl4030-madc-hwmon.c:123:12: warning: ‘twl4030_madc_hwmon_remove’ defined
but not used [-Wunused-function]

Fix by removing __exit_p as well.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <khali@linux-fr.org>