If __ethtool_get_link_ksettings() is failed and with
non-zero value, prb_calc_retire_blk_tmo() should return
DEFAULT_PRB_RETIRE_TOV firstly.

This patch is to refactory code and make it more readable.
Signed-off-by: NMao Wenan <maowenan@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>