ata: include: libata: Move fields commonly over-written to separate MACRO

This is a pre-cursor to some upcoming W=1 fix-ups.

Fixes the following W=1 kernel build warning(s):

Cc: Jens Axboe <axboe@kernel.dk>
Cc: Mark Lord <mlord@pobox.com>
Cc: Philipp Zabel <p.zabel@pengutronix.de>
Cc: Tejun Heo <tj@kernel.org>
Cc: linux-ide@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210528090502.1799866-2-lee.jones@linaro.org


Signed-off-by: Jens Axboe <axboe@kernel.dk>