igb: Refactor igb_configure_cbs()

Make this function retrieve what it needs from the Tx ring being
addressed since it already relies on what had been saved on it before.
Also, since this function will be used by the upcoming Launchtime
patches rename it to better reflect its intention. Note that
Launchtime is not part of what 802.1Qav specifies, but the i210
datasheet refers to this set of functionality as "Qav Transmission
Mode".

Here we also perform a tiny refactor at is_any_cbs_enabled(), and add
further documentation to igb_setup_tx_mode().

Signed-off-by: Jesus Sanchez-Palencia <jesus.sanchez-palencia@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>