ide-pmac: add ->cable_detect method

Add ->cable_detect method and remove no longer needed pmif->cable_80 flag
(there is also no need to mask ->udma_mask now).

This fixes:

- forced ignoring of cable detection (needed for some CF devices & debug)

- cable detection for warm-plug

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>