ide: move ide_pio_cycle_time() to ide-timings.c

All ide_pio_cycle_time() users already select CONFIG_IDE_TIMINGS
so move the function from ide-lib.c to ide-timings.c.

While at it:

- convert ide_pio_cycle_time() to use ide_timing_find_mode()

- cleanup ide_pio_cycle_time() a bit

There should be no functional changes caused by this patch.

Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>