net: hix5hd2_gmac: avoid integer overload warning

BITS_RX_EN is an 'unsigned long' constant, so the ones complement of that
has bits set that do not fit into a 32-bit variable on 64-bit architectures,
which causes a harmless gcc warning:

drivers/net/ethernet/hisilicon/hix5hd2_gmac.c: In function 'hix5hd2_port_disable':
drivers/net/ethernet/hisilicon/hix5hd2_gmac.c:374:2: warning: large integer implicitly truncated to unsigned type [-Woverflow]
  writel_relaxed(~(BITS_RX_EN | BITS_TX_EN), priv->base + PORT_EN);

This adds a cast to (u32) to tell gcc that the code is indeed fine.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>