r8152: add a helper function about setting EEE

Add a helper function "rtl_eee_enable" for setting EEE. Besides, I
move r8153_eee_en() and r8153b_eee_en(). And, I remove r8152b_enable_eee(),
r8153_set_eee(), and r8153b_set_eee().

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>