net: ethernet: ti: cpdma: fix desc re-queuing

The currently processing cpdma descriptor with EOQ flag set may
contain two values in Next Descriptor Pointer field:
- valid pointer: means CPDMA missed addition of new desc in queue;
- null: no more descriptors in queue.
In the later case, it's not required to write to HDP register, but now
CPDMA does it.

Hence, add additional check for Next Descriptor Pointer != null in
cpdma_chan_process() function before writing in HDP register.

Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>