pata_legacy: fix io/irq mismatch

pata_legacy fails to detect the disk on my old ISA/VLB 486:
it starts to probe io=0x1f0 ctr=0x3f6 irq=15, complains
loudly about IDENTIFYs timing out, and finally fails.
(Sorry I couldn't capture the kernel's boot messages.)

It turns out that the driver's mapping from io to irq in
legacy_irq[] is wrong: index 0 for io=0x1f0 has irq=15 but
should have irq=14, and index 1 for io=0x170 has irq=14 but
should have irq=15. This is confirmed by a comparison with
include/asm-i386/ide.h:ide_default_irq().

This patch swaps the first two elements in legacy_irq[],
which makes pata_legacy work on my 486.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Jeff Garzik <jeff@garzik.org>