ide-floppy: Fix unformatted media crash

A ZIP or similar with unformatted media will cause crashes when attempts
are made to read/write it in some cases. This is because bs_factor is
zero and we divide by it causing an oops.

As the size of a non-accessible/non-existant media is really a bit of a
zen question it doesn't matter if non-existant media is 512 bytes per
sector or zero. Setting it to 1 causes us to generate 512 bytes/sector
accesses and error properly.

Based on a fix found lurking in an ancient bugzilla entry since about 2004 (ugghhh)

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>