axgbe: Various stability improvements

Hook in RSS glue.

Default to "off" for the split header feature to ensure netmap
compatibility.

Change the PCS indirection register values based on hardware type
(ported from Linux).

Move tunable settings to sysctl_init() and set the defaults there.
Ensure it's called at the right time by moving it back.

Reset PHY RX data path when mailbox command times out (Ported from
Linux).

Check if VLAN HW tagging is enabled before assuming a VLAN tag
is present in a descriptor.

Disable the hardware filter since multicast traffic is dropped
in promisc mode.

Remove unnecessary return statement.

Missing sfp_get_mux, causing a race between ports to read
SFP(+) sideband signals.

Validate and fix incorrectly initialized polarity/configuration
registers.

Remove unnecessary SFP reset.

axgbe_isc_rxd_pkt_get has no error state, remove unnecessary
big packet check.

Enable RSF to prevent zero-length packets while in Netmap mode.

DMA cache coherency update (ported from Linux).

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1103