aec62xx: remove init_dma() method (take 2)

Get rid of the init_dma() method (which had no particular reason to exist) by
folding it into the init_hwif() method. While at it, also perform some cleanup
in the latter method:

- get rid of the useless clearing of hwif->autodma;

- fold the serialization code into one 'if' statement;

- fold setting the drives' 'autotune' and 'autodma' fields into the single
  statements...

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