sysfs: error handling in sysfs, fill_read_buffer()

if a driver returns an error in fill_read_buffer(), the buffer will be
marked as filled. Subsequent reads will return eof. But there is
no data because of an error, not because it has been read.
Not marking the buffer filled is the obvious fix.

Signed-off-by: Oliver Neukum <oliver@neukum.name>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>