PCI: Make minimum bridge window alignment reference more obvious

Calculations related to bridge window size contain literal 20 that is the
minimum alignment for a bridge window. Make the code more obvious by
converting the literal 20 to __ffs(SZ_1M).

Link: https://lore.kernel.org/r/20240507102523.57320-8-ilpo.jarvinen@linux.intel.com


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
[bhelgaas: squash https://lore.kernel.org/r/20240612093250.17544-1-ilpo.jarvinen@linux.intel.com

]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>