dma: bcm6348: incorrect buffer allocation

Calling calloc() for 0 members does not make any sense.
Setting ch_priv->busy_desc = NULL for ch_priv->desc_cnt > 0 is equally
unreasonable.

The current code will lead to a NULL dereference in bcm6348_iudma_enable().

The assignments for ch_priv->busy_desc are obviously swapped.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>