hw/riscv: virt: Drop the 'link_up' parameter of gpex_pcie_init()

hw/riscv: virt: Drop the 'link_up' parameter of gpex_pcie_init()

`link_up` is never used in gpex_pcie_init(). Drop it.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210220144807.819-3-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>