staging: mt7621-pci: add clarification comment in 'mt7621_pcie_init_virtual_bridges'

Add a comment explaining a bit what is going on in this function.
It would be helfulp for other people for a better understanding
of the code.

Suggested-by: NeilBrown <neil@brown.name>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Link: https://lore.kernel.org/r/20200413155018.31921-2-sergio.paracuellos@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>