ide-floppy: use scatterlists for pio transfers

Use hwif->sg_table for pio transfers instead of fumbling
with block layer internals in the driver. Also, make debug
statements more informative in .._do_request() while at it.

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
[bart: fixup pc->b_count in idefloppy_blockpc_cmd()]
[bart: add missing include (noticed by Stephen Rothwell)]
[bart: map hwif->sg_{table,nents} on pc->{sg,sg_cnt} (multi-IRQs-per-sg fix)]
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>