core: Fix calculation of MMCONFIG region size

The PCI Firmware Specification says: "For PCI-X and PCI Express
platforms utilizing the enhanced configuration access method, the base
address of the memory mapped configuration space always corresponds to
bus number 0 (regardless of the start bus number decoded by the host
bridge) [...]." So drop the start bus from the size calculation.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>