net: stmmac: Avoid DMA_CHAN_CONTROL write if no Split Header support

The driver assumes that split headers can be enabled/disabled without
stopping/starting the device, so it writes DMA_CHAN_CONTROL from
stmmac_set_features().  However, on my system (IP v5.10a without Split
Header support), simply writing DMA_CHAN_CONTROL when DMA is running
(for example, with the commands below) leads to a TX watchdog timeout.

 host$ socat TCP-LISTEN:1024,fork,reuseaddr - &
 device$ ethtool -K eth0 tso off
 device$ ethtool -K eth0 tso on
 device$ dd if=/dev/zero bs=1M count=10 | socat - TCP4:host:1024
 <tx watchdog timeout>

Note that since my IP is configured without Split Header support, the
driver always just reads and writes the same value to the
DMA_CHAN_CONTROL register.

I don't have access to any platforms with Split Header support so I
don't know if these writes to the DMA_CHAN_CONTROL while DMA is running
actually work properly on such systems.  I could not find anything in
the databook that says that DMA_CHAN_CONTROL should not be written when
the DMA is running.

But on systems without Split Header support, there is in any case no
need to call enable_sph() in stmmac_set_features() at all since SPH can
never be toggled, so we can avoid the watchdog timeout there by skipping
this call.

Fixes: 8c6fc097

 ("net: stmmac: gmac4+: Add Split Header support")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: David S. Miller <davem@davemloft.net>