ath10k: wmi: remove array of flexible structures

I updated to sparse v0.6.4 and it warns:

drivers/net/wireless/ath/ath10k/wmi.c: note: in included file (through drivers/net/wireless/ath/ath10k/core.h):
drivers/net/wireless/ath/ath10k/wmi.h:3481:34: warning: array of flexible structures

Fix it by changing the type to u8 array, in struct wmi_phyerr_hdr_arg it's
stored as a void pointer anyway.

Compile tested only.

Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20211209091545.6098-2-kvalo@kernel.org