Modify PCI Bridge Control ISA flag for clarity

This patch changes PCI_BRIDGE_CTL_NO_ISA to PCI_BRIDGE_CTL_ISA
and modifies it's clarifying comment and locations where used.
The change reduces the chance of future confusion since it makes
the set/unset meaning of the bit the same in both the bridge
control register and bridge_ctl field of the pci_bus struct.
Signed-off-by: NGary Hade <garyhade@us.ibm.com>
Acked-by: NLinas Vepstas <linas@austin.ibm.com>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>