drivers: net: silence compiler warning in smc91x.c

If used 64 bit compiler GCC warns that:

drivers/net/ethernet/smsc/smc91x.c:1897:7:
warning: cast from pointer to integer of different
size [-Wpointer-to-int-cast]

This patch fixes this by changing typecast from "unsigned int" to "unsigned long"

CC: "David S. Miller" <davem@davemloft.net>
CC: Jingoo Han <jg1.han@samsung.com>
CC: netdev@vger.kernel.org
Signed-off-by: Pankaj Dubey <pankaj.dubey@samsung.com>
Signed-off-by: David S. Miller <davem@davemloft.net>