ataflop: use a statically allocated error counters

This is the last driver making use of fd_request->error_count, which is
easy to get wrong as was shown in floppy.c.  We don't need to keep it
there, it can be moved to the atari_floppy_struct instead, so let's do
this.

Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Minh Yuan <yuanmingbuaa@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>