wifi: mt76: mt7996: Initialize hdr before passing to skb_put_data()

A new warning in clang [1] points out a couple of places where a hdr
variable is not initialized then passed along to skb_put_data().

  drivers/net/wireless/mediatek/mt76/mt7996/mcu.c:1894:21: warning: variable 'hdr' is uninitialized when passed as a const pointer argument here [-Wuninitialized-const-pointer]
   1894 |         skb_put_data(skb, &hdr, sizeof(hdr));
        |                            ^~~
  drivers/net/wireless/mediatek/mt76/mt7996/mcu.c:3386:21: warning: variable 'hdr' is uninitialized when passed as a const pointer argument here [-Wuninitialized-const-pointer]
   3386 |         skb_put_data(skb, &hdr, sizeof(hdr));
        |                            ^~~

Zero initialize these headers as done in other places in the driver when
there is nothing stored in the header.

Cc: stable@vger.kernel.org
Fixes: 98686cd2 ("wifi: mt76: mt7996: add driver for MediaTek Wi-Fi 7 (802.11be) devices")
Link: https://github.com/llvm/llvm-project/commit/00dacf8c22f065cb52efb14cd091d441f19b319e [1]
Closes: https://github.com/ClangBuiltLinux/linux/issues/2104


Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20250715-mt7996-fix-uninit-const-pointer-v1-1-b5d8d11d7b78@kernel.org


Signed-off-by: Felix Fietkau <nbd@nbd.name>