alim15x3: remove redundant m5229_revision check

init_dma_ali15x3() guarantees that hwif->dma_base will never be set for
m5229_revision < 0x20 so remove redundant m5229_revision >= 0x20 check from
init_hwif_common_ali15x3().

While at it remove incorrect comment.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>