Don't enable I/O or memory mode in a device's command register if the BAR

Don't enable I/O or memory mode in a device's command register if the BAR
we are processing has a base address of zero.  Note that this will only
change behavior for devices where all the BARs of a given type have a base
address of 0 since we will enable the appropriate access when we encounter
the first BAR with a base that is not 0.  Specifically, this allows certain
Toshiba laptops to no longer require 'hw.pci.enable_io_modes=0' to avoid
hangs during boot.

PR:		kern/20040
PR:		i386/63776 (possibly)
PR:		i386/68900 (possibly)
PR:		i386/74532 (possibly)
MFC after:	1 week