staging: mt7621-pci: fix hang when nothing is connected to pcie ports

When nothing is connected to pcie ports, each port is set to reset state.
When this occurs, next access result in a hang on boot as follows:

mt7621-pci 1e140000.pcie: pcie0 no card, disable it (RST & CLK)
mt7621-pci 1e140000.pcie: pcie1 no card, disable it (RST & CLK)
mt7621-pci 1e140000.pcie: pcie2 no card, disable it (RST & CLK)
[ HANGS HERE ]

Fix this just detecting 'nothing is connected state' to avoid next accesses
to pcie port related configuration registers.

Fixes: b99cc3a2 ("staging: mt7621-pci: avoid custom 'map_irq' function")
Cc: stable <stable@vger.kernel.org>
Reported-by: DENG Qingfang <dqfext@gmail.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20210823170803.2108-1-sergio.paracuellos@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/mt7621-pci/pci-mt7621.c

@@ -56,6 +56,7 @@
 #define PCIE_BAR_ENABLE			BIT(0)
 #define PCIE_PORT_INT_EN(x)		BIT(20 + (x))
 #define PCIE_PORT_LINKUP		BIT(0)
+#define PCIE_PORT_CNT			3
 
 #define PERST_DELAY_MS			100
 
@@ -388,10 +389,11 @@ static void mt7621_pcie_reset_ep_deassert(struct mt7621_pcie *pcie)
 	msleep(PERST_DELAY_MS);
 }
 
-static void mt7621_pcie_init_ports(struct mt7621_pcie *pcie)
+static int mt7621_pcie_init_ports(struct mt7621_pcie *pcie)
 {
 	struct device *dev = pcie->dev;
 	struct mt7621_pcie_port *port, *tmp;
+	u8 num_disabled = 0;
 	int err;
 
 	mt7621_pcie_reset_assert(pcie);
@@ -423,6 +425,7 @@ static void mt7621_pcie_init_ports(struct mt7621_pcie *pcie)
 				slot);
 			mt7621_control_assert(port);
 			port->enabled = false;
+			num_disabled++;
 
 			if (slot == 0) {
 				tmp = port;
@@ -433,6 +436,8 @@ static void mt7621_pcie_init_ports(struct mt7621_pcie *pcie)
 				phy_power_off(tmp->phy);
 		}
 	}
+
+	return (num_disabled != PCIE_PORT_CNT) ? 0 : -ENODEV;
 }
 
 static void mt7621_pcie_enable_port(struct mt7621_pcie_port *port)
@@ -540,7 +545,11 @@ static int mt7621_pci_probe(struct platform_device *pdev)
 		return err;
 	}
 
-	mt7621_pcie_init_ports(pcie);
+	err = mt7621_pcie_init_ports(pcie);
+	if (err) {
+		dev_err(dev, "Nothing connected in virtual bridges\n");
+		return 0;
+	}
 
 	err = mt7621_pcie_enable_ports(bridge);
 	if (err) {