Tofino PCIe link status should tell us more than a bool

[rmustacc]

Currently the code to tell if a PCIe link is up looks something like this:

    /// Poll the PCIe Dev Info register to determine if the PCIe link with a
    /// host is up. Note that this function should only be called when Tofino is
    /// in A0, otherwise it may cause the debug port in the mainboard controller
    /// to get stuck.
    pub fn pcie_link_up(&mut self) -> Result<bool, SeqError> {
        // There is no bit description in the documentation available for this
        // register, so make use of observed magic values.
        Ok(self.debug_port.read_direct(
            DirectBarSegment::Bar0,
            TofinoBar0Registers::PcieDevInfo,
        )? & 0xf 
            == 0xf)
    }   

Even if I don't know the meaning of all the bits, when we have cases where two ends disagree if the link is up, it'd be helpful to see if this was 0 or some of the bits are there rather than just false.