staging: mt7621-pci: factor out mt7621_pci_get_cfgaddr function

To get config address the same pattern is repeated in some functions
along the code. Factor out a new mt7621_pci_get_cfgaddr for calculate
it and use it in convenient places. Adjust types to match to u32 where
is neccesary.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>