eth: tg3: silence the GCC 12 array-bounds warning

GCC 12 currently generates a rather inconsistent warning:

drivers/net/ethernet/broadcom/tg3.c:17795:51: warning: array subscript 5 is above array bounds of ‘struct tg3_napi[5]’ [-Warray-bounds]
17795 |                 struct tg3_napi *tnapi = &tp->napi[i];
      |                                           ~~~~~~~~^~~

i is guaranteed < tp->irq_max which in turn is either 1 or 5.
There are more loops like this one in the driver, but strangely
GCC 12 dislikes only this single one.

Silence this silliness for now.
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/Makefile

@@ -17,3 +17,8 @@ obj-$(CONFIG_BGMAC_BCMA) += bgmac-bcma.o bgmac-bcma-mdio.o
 obj-$(CONFIG_BGMAC_PLATFORM) += bgmac-platform.o
 obj-$(CONFIG_SYSTEMPORT) += bcmsysport.o
 obj-$(CONFIG_BNXT) += bnxt/
+
+# FIXME: temporarily silence -Warray-bounds on non W=1+ builds
+ifndef KBUILD_EXTRA_WARN
+CFLAGS_tg3.o += -Wno-array-bounds
+endif