hwmon: (it87) Delete pwmN_freq files on driver removal

In commit f8d0c19a I forgot to delete
the pwmN_freq files on driver removal, here's the fix.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Riku Voipio <riku.voipio@movial.fi>
Signed-off-by: Mark M. Hoffman <mhoffman@lightlink.com>