staging: iio: io-trig-bfin-timer: use dev_get_platdata()

Use dev_get_platdata() for retrieving the platform data instead of
accessing dev->platform_data directly. Move the assignment out of
the declaration (avoid lines over 80 char and put it close to
usage).

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>