staging: mt7621-pci: add comment clarifying inverted reset lines

To avoid people reading this code being very confused, add a comment
clarifying the need for invert resets on some chip revisions.

Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>