wifi: brcmfmac: Use struct_size() in code ralated to struct brcmf_dload_data_le

Prefer struct_size() over open-coded versions of idiom:

sizeof(struct-with-flex-array) + sizeof(typeof-flex-array-elements) * count

where count is the max number of items the flexible array is supposed to
contain.

In this particular case, in the open-coded version sizeof(typeof-flex-array-elements)
is implicit in _count_ because the type of the flex array data is u8:

drivers/net/wireless/broadcom/brcm80211/brcmfmac/fwil_types.h:941:
 941 struct brcmf_dload_data_le {
 942         __le16 flag;
 943         __le16 dload_type;
 944         __le32 len;
 945         __le32 crc;
 946         u8 data[];
 947 };

Link: https://github.com/KSPP/linux/issues/160
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/41845ad3660ed4375f0c03fd36a67b2e12fafed5.1668548907.git.gustavoars@kernel.org