ixgbe: add methods for combined read and write operations

Some X550 procedures will be using CS4227 PHY and need to
perform combined read and write operations.  This patch
adds those methods.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>