Fix dumpfs kernel.

case 1:
	IDE0=primary=disk
	IDE1=primary=disk
	IDE1=secondary=disk

	filsys main ch0f{h2h3}
	-> panic: ataxfer: sdunits[3].subno is 0, not 1

case 2:
	install SATA-RAID cards. this card has 4port 2controllers.
	card1=ctlr0=port0=disk
	card1=ctlr1=port0=disk
	card2=ctlr0=port0=disk

	auto detect:
		ata h4 ...
		ata h4 ...
		ata h4 ...

	all h4?

	filsys main ch4f{h6h8}
	-> panic: ataxfer: sdunits[4].dev=$p1 is wrong controller (want $p2)