Zero-length arrays as fake flexible arrays are deprecated and we are
moving towards adopting C99 flexible-array members instead.

Use the DECLARE_FLEX_ARRAY() helper macro to transform zero-length
arrays in unions with flexible-array members.

Address the following warning found with GCC-13 and
-fstrict-flex-arrays=3 enabled:
drivers/iio/accel/cros_ec_accel_legacy.c:66:46: warning: array subscript <unknown> is outside array bounds of ‘struct ec_response_motion_sensor_data[0]’ [-Warray-bounds=]

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/21
Link: https://github.com/KSPP/linux/issues/193
Link: https://github.com/KSPP/linux/issues/262
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Tzung-Bi Shih <tzungbi@kernel.org>
Link: https://lore.kernel.org/r/ZAZUGBmSLc5wg7AK@work