net: stmmac: dwxgmac2: Do not disable whole RX in dma_stop_rx()

We don't need to disable the whole RX when dma_stop_rx() is called
because there may be the need of just disabling 1 DMA channel.

This is also needed for stmmac Flow Control selftest.

Signed-off-by: Jose Abreu <joabreu@synopsys.com>
Cc: Joao Pinto <jpinto@synopsys.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Signed-off-by: David S. Miller <davem@davemloft.net>