net: designware: set the PS bit when resetting DMA bus in MII configuration

On the SPEAr600 SoC, which has the dwmac1000 variant of the IP block,
the DMA reset never succeeds when a MII PHY is used (no problem with a
GMII PHY). The designware_eth_init() function sets the
DMAMAC_SRST bit in the DMA_BUS_MODE register, and then
polls until this bit clears. When a MII PHY is used, with the current
driver, this bit never clears and the driver therefore doesn't work.

The reason is that the PS bit of the GMAC_CONTROL register should be
correctly configured for the DMA reset to work. When the PS bit is 0,
it tells the MAC we have a GMII PHY, when the PS bit is 1, it tells
the MAC we have a MII PHY.

Doing a DMA reset clears all registers, so the PS bit is cleared as
well. This makes the DMA reset work fine with a GMII PHY. However,
with MII PHY, the PS bit should be set.

We have identified this issue thanks to two SPEAr600 platform:

- One equipped with a GMII PHY, with which the existing driver was
working fine.

- One equipped with a MII PHY, where the current driver fails because
the DMA reset times out.

Note: Taken from https://www.spinics.net/lists/netdev/msg432578.html

Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>

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