wifi: mt76: mt7921: Replace fake flex-arrays with flexible-array members

wifi: mt76: mt7921: Replace fake flex-arrays with flexible-array members

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:
drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:266:25: warning: array subscript 0 is outside array bounds of ‘struct mt7921_asar_dyn_limit_v2[0]’ [-Warray-bounds=]
drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:263:25: warning: array subscript 0 is outside array bounds of ‘struct mt7921_asar_dyn_limit[0]’ [-Warray-bounds=]
drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:223:28: warning: array subscript <unknown> is outside array bounds of ‘struct mt7921_asar_geo_limit_v2[0]’ [-Warray-bounds=]
drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:220:28: warning: array subscript <unknown> is outside array bounds of ‘struct mt7921_asar_geo_limit[0]’ [-Warray-bounds=]
drivers/net/wireless/mediatek/mt76/mt7921/acpi_sar.c:334:37: warning: array subscript i is outside array bounds of ‘u8[0]’ {aka ‘unsigned char[]’} [-Warray-bounds=]

Notice that the DECLARE_FLEX_ARRAY() helper allows for flexible-array
members in unions.

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/272
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>