sysfs.h: don't return a void-valued expression in sysfs_remove_file

Sparse was complaining about that:
include/linux/sysfs.h:432:9: warning: returning void-valued expression

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>