wcn36xx: Avoid memset() beyond end of struct field

In preparation for FORTIFY_SOURCE performing compile-time and run-time
field bounds checking for memset(), avoid intentionally writing across
neighboring array fields.

Instead of writing past the end of the header to reach the rest of
the body, replace the redundant function with existing macro to wipe
struct contents and set field values. Additionally adjusts macro to add
missing parens.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210617171058.3410494-1-keescook@chromium.org