include/hw/pci/pcie_host: Correct PCIE_MMCFG_BUS_MASK

include/hw/pci/pcie_host: Correct PCIE_MMCFG_BUS_MASK

According to [1] address bits 27 - 20 are mapped to the bus number (the
TLPs bus number field is 8 bits). Below is the formula taken from Table
7-1 in [1].

"
Memory Address | PCI Express Configuration Space
A[(20+n-1):20] | Bus Number, 1 ≤ n ≤ 8
"

[1] PCI Express® Base Specification Revision 5.0 Version 1.0

Signed-off-by: Francisco Iglesias <frasse.iglesias@gmail.com>
Message-Id: <20220411221836.17699-2-frasse.iglesias@gmail.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>