iavf: change the flex-byte support number to macro definition

The maximum number (2) of flex-byte support is derived from ethtool
use-def data size (8 byte).

Change the magic number 2 to macro definition, and add the comment to
track the design thinking, so the code is clear and easily maintained.

Signed-off-by: Haiyue Wang <haiyue.wang@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>