ide-floppy: use drive->pc_callback instead of pc->callback

It is important that drive->pc_callback is set prior to enabling IRQs on the
device since this is called from the IRQ handler. Otherwise it hurts as I learnt
the hard way from the several "Kernel panic - not synching: Fatal exception in
interrupt" during the weekend :).

The if-else block in the IRQ handler is only temporary so that bisect searches
don't break and it'll be removed after converting the remainder of the drivers.

There should be no functionality change resulting from this patch.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>