ide-floppy: factor out ioctl handlers from idefloppy_ioctl()

By passing idefloppy_floppy_t *floppy to the factored out functions, we get
rid of (almost) all local vars so stack usage should be at minimum here. Also,
we merge idefloppy_begin_format() into idefloppy_format_start() since it is its
only user. Also, rename idefloppy_format_start() to idefloppy_format_unit().
Finally, rename the reworked functions to ide_floppy..().

Bart
- minor CodingStyle fixup

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