net: dwc_eth_qos: Initialize the transmit tail pointer in eqos_start()

net: dwc_eth_qos: Initialize the transmit tail pointer in eqos_start()

The DesignWare Cores Ethernet Quality-of-Service databook state that
descriptors up to one location less than the one indicated by the
descriptor tail pointer are owned by the DMA. The DMA continues to
process the descriptors until the following condition occurs:

  Current Descriptor Pointer == Descriptor Tail Pointer

The DMA goes into suspend mode when this condition occurs, and updating
the tail pointer resume the DMA processing.

Configure the transmit tail pointer to the first (current) descriptor
pointer so that the tail pointer is a valid address instead of being
initialized to NULL when transmit DMA is started.

Also update the receive tail pointer comment to state that by pointing
to the last descriptor we are actually implying that all receive
descriptors are owned by and can be processed by the DMA.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>