piix: tuneproc() fixes/cleanups

Fix/cleanup the driver's tuneproc() and ratemask() methods:

- PPE, IE, and TIME bits need to be cleared beforehand for the slave drive as
  well as master (Alan probably just forgot about it);

- this driver only supports PIO modes up to 4, so must pass the correct limit
  to ide_get_best_pio_mode();

- use min_t() macro instead of min();

- simplify slave vs master drive evaluation;

- do come coding and formatting cleanups...

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>