net/ixgbe/base: support X550em_a SGMII FC autoneg

net/ixgbe/base: support X550em_a SGMII FC autoneg

Create flow control (FC) setup and enable functions to configure PHY
registers. These registers advertise FC settings according to the user
requested mode. On link establishment, these settings are negotiated
with the link partner to agree on the settings to use: RX and TX enabled,
TX only enabled, or FC disabled and RX enabled (which also enables TX).

Remove the part of code that performs FC auto-negotiation (AN) in
ixgbe_setup_m88 and put it in a new function to maintain consistency
with other FC AN implementations so far. Add function pointers to the
setup-fc and enable-fc functions so that they get called during init
to perform FC AN.

Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
Acked-by: Wenzhuo Lu <wenzhuo.lu@intel.com>