net/ixgbe/base: add write flush required by Inphi PHY

This patch updates the configuration of PHY from Inphi (www.inphi.com)
to flush the register write with a reg read.
The Inphi PHY is configured in ixgbe_setup_mac_link_sfp_x550a.
The Inphi PHY setup flow has been updated to read configuration reg,
write only linear/non-linear, and then read (write flush).

Signed-off-by: Wei Dai <wei.dai@intel.com>