pmac: remove needless pmac_ide_destroy_dmatable() wrapper

hwif->sg_nents is always != 0 when this function is called
and there is also no need to explicitely zero hwif->sg_nents.

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