ath6kl: Use struct_group() to avoid size-mismatched casting

ath6kl: Use struct_group() to avoid size-mismatched casting

In builds with -Warray-bounds, casts from smaller objects to larger
objects will produce warnings. These can be overly conservative, but since
-Warray-bounds has been finding legitimate bugs, it is desirable to turn
it on globally. Instead of casting a u32 to a larger object, redefine
the u32 portion of the header to a separate struct that can be used for
both u32 operations and the distinct header fields. Silences this warning:

drivers/net/wireless/ath/ath6kl/htc_mbox.c: In function 'htc_wait_for_ctrl_msg':
drivers/net/wireless/ath/ath6kl/htc_mbox.c:2275:20: error: array subscript 'struct htc_frame_hdr[0]' is partly outside array bounds of 'u32[1]' {aka 'unsigned int[1]'} [-Werror=array-bounds]
 2275 |         if (htc_hdr->eid != ENDPOINT_0)
      |                    ^~
drivers/net/wireless/ath/ath6kl/htc_mbox.c:2264:13: note: while referencing 'look_ahead'
 2264 |         u32 look_ahead;
      |             ^~~~~~~~~~

This change results in no executable instruction differences.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20211207063538.2767954-1-keescook@chromium.org