net: stmmac: xgmac: Fix RSS writing wrong keys

Commit b6b6cc9a, changed the call to dwxgmac2_rss_write_reg()
passing it the variable cfg->key[i].

As key is an u8 but we write 32 bits at a time we need to cast it into
an u32 so that the correct key values are written. Notice that the for
loop already takes this into account so we don't try to write past the
keys size.

Fixes: b6b6cc9a

 ("net: stmmac: selftest: avoid large stack usage")
Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>