floppy: use warning macros

Convert assertions to use WARN().  There are several error checks in the
code for things that should never happen.  Convert them to standard
warnings so kerneloops.org will see them.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>