amd-xgbe: Rename pre_xmit function to dev_xmit

The pre_xmit function name implies that it performs operations prior
to transmitting the packet when in fact it is responsible for setting
up the descriptors and initiating the transmit.  Rename this to
function from pre_xmit to dev_xmit, which is consistent with the name
used during receive processing - dev_read.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>