staging: mt7621-pci: reverse condition to check for enabled port

Each enabled port is being checked in 'mt7621_pcie_enable_ports"
function calling 'mt7621_pcie_enable_port'. The return value for
this function on success is zero, so the check is reversed. Fix it.

Fixes: 802a2f7b

: staging: mt7621-pci: factor out 'mt7621_pcie_enable_port' function
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>