pwm: cros_ec: Rename "priv_auto_alloc_size" to "priv_auto"

With commit 41575d8e ("dm: treewide: Rename auto_alloc_size members
to be shorter") "priv_auto_alloc_size" was renamed to "priv_auto". This
driver was sent to the mailing list before that change, merged after it,
and still has the old form. Apply the rename here as well.

Fixes: 1b9ee288 ("pwm: Add a driver for Chrome OS EC PWM")
Signed-off-by: Alper Nebi Yasak <alpernebiyasak@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>