net: stmmac: Remove uneeded check for GMAC version in stmmac_xmit

We either have .enable_dma_transmission or .set_tx_tail_ptr in the HW
table callbacks, we can never have both so there is no need to check for
GMAC version.

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