dmaengine: shdma: tidyup spin_unlock_bh on sh_chan_xfer_ld_queue

It is not readable that there is any spin_unlock_bh on same function.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>