ide-floppy: factor out generic disk handling code to ide-gd-floppy.c

While at it:
- idefloppy_do_request() -> ide_floppy_do_request()
- idefloppy_end_request() -> ide_floppy_end_request()
- idefloppy_setup() -> ide_floppy_setup()

There should be no functional changes caused by this patch.

Acked-by: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>