floppy: use symbolic register names in the m68k port

Now we can use FD_STATUS and FD_DATA instead of 4 or 5, let's do
this, and also use STATUS_DMA and STATUS_READY for the status bits.

Link: https://lore.kernel.org/r/20200331094054.24441-4-w@1wt.eu


Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Denis Efremov <efremov@linux.com>