w83791d: Fix unchecked return status

Replace all unchecked calls to device_create_file with a single group
declaration, and one call to sysfs_create_group, and check that one
return status. Also remove the files on device detach.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Signed-off by: Charles Spirakis <bezaur@gmail.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>