igb: remove never changed variable `ret_val'

The variable used for return status in `igb_write_xmdio_reg' function
is never changed  and this function is just need return 0. Thus, the
`ret_val' can be removed and return 0 at the end of the
`igb_write_xmdio_reg' function.

Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>