staging: iio: trigger: blackfin timer: replace device_attr with device_attr_rw

This is a clean-up patch which replaces DEVICE_ATTR() macro with the
file permission specific DEVICE_ATTR_RW() macro for compaction and
readability. Done using coccinelle.

Signed-off-by: Aishwarya Pant <aishpant@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>