net: stmmac: arrange Tx tail pointer update to stmmac_flush_tx_descriptors

This patch organizes TX tail pointer update into a new function called
stmmac_flush_tx_descriptors() so that we can reuse it in stmmac_xmit(),
stmmac_tso_xmit() and up-coming XDP implementation.

Changes to v2:
 - Fix for warning: unused variable ‘desc_size’
   https://patchwork.hopto.org/static/nipa/457321/12170149/build_32bit/stderr



Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>