igb: dont drop packets if rx flow control is enabled

If Rx flow control has been enabled (via autoneg or forced), packets
should not be dropped due to Rx descriptor ring exhaustion. Instead
pause frames should be used to apply back pressure. This only applies
if VFs are not in use.

Move SRRCTL setup to its own function for easy reuse and only set drop
enable bit if Rx flow control is not enabled.

Since v1: always enable dropping of packets if VFs in use.

Signed-off-by: Robert Beckett <bob.beckett@collabora.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>