wifi: carl9170: Replace fake flex-array with flexible-array member

wifi: carl9170: Replace fake flex-array with flexible-array member

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

Address the following warnings found with GCC-13 and
-fstrict-flex-arrays=3 enabled:
include/shared/tx.c:702:61: warning: array subscript i is outside array bounds of ‘const struct _carl9170_tx_status[0]’ [-Warray-bounds=]
include/shared/tx.c:701:65: warning: array subscript i is outside array bounds of ‘const struct _carl9170_tx_status[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/267
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>