Staging: iio: add file pointer to sysfs callbacks

The sysfs attribute call backs take a file pointer these days.  This was
added in 2c3c8bea "sysfs: add struct file* to bin_attr callbacks"

Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>