Use the FIELD_PREP() helper, instead of open-coding the same operation.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/f8831b88346b36fc6e01e0910d0db6c94287d2b4.1637593297.git.geert+renesas@glider.beSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>