fdc: take side count into account

fdc: take side count into account

Floppies can be simple or double-sided. However, current code
was only taking the common case into account (ie 2 sides).

This repairs single-sided floppies, which where totally broken
before this patch : for track > 0, wrong sector number was
calculated, and data was read/written at wrong place on
underlying device.

Fortunately, only some 360 kB floppies are single-sided, so
this bug was probably not seen much.

Signed-off-by: Hervé Poussineau <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>