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

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: b99cc3a2b6b6 ("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.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>