mmc: dw_mmc: fix falling from idmac to PIO mode when dw_mci_reset occurs

It was found that in IDMAC mode after soft-reset driver switches
to PIO mode.

That's what happens in case of DTO timeout overflow calculation failure:
1. soft-reset is called
2. driver restarts dma
3. descriptors states are checked, one of descriptor is owned by the IDMAC.
4. driver can't use DMA and then switches to PIO mode.

Failure was already fixed in:
https://www.spinics.net/lists/linux-mmc/msg48125.html

.

Behaviour while soft-reset is not something we except or
even want to happen. So we switch from dw_mci_idmac_reset
to dw_mci_idmac_init, so descriptors are cleaned before starting dma.

And while at it explicitly zero des0 which otherwise might
contain garbage as being allocated by dmam_alloc_coherent().

Signed-off-by: Evgeniy Didin <Evgeniy.Didin@synopsys.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Cc: Shawn Lin <shawn.lin@rock-chips.com>
Cc: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Cc: linux-snps-arc@lists.infradead.org
Cc: <stable@vger.kernel.org> # 4.4+
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>