usb/net/r8152: fix integer overflow in expression

config: make ARCH=avr32 allyesconfig
drivers/net/usb/r8152.c: In function 'rtl8152_start_xmit':
drivers/net/usb/r8152.c:956: warning: integer overflow in expression

   955	memset(tx_desc, 0, sizeof(*tx_desc));
 > 956	tx_desc->opts1 = cpu_to_le32((len & TX_LEN_MASK) | TX_FS | TX_LS);
   957	tp->tx_skb = skb;

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Spotted-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>