net: stmmac: xgmac: Fix RSS not writing all Keys to HW

The sizeof(cfg->key) is != ARRAY_SIZE(cfg->key). Fix it. This warning is
triggered when running with cc flag -Wsizeof-array-div.

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Fixes: 76067459

 ("net: stmmac: Implement RSS and enable it in XGMAC core")
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>