PCI: Fix calculation of bridge window's size and alignment

In case that one device's alignment is greater than its size, we may
get an incorrect size and alignment for its bus's memory window in
pbus_size_mem().  Fix this case.

Signed-off-by: Yongji Xie <elohimes@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>