r8169: fix broken ring index handling in suspend/resume

rtl8169_hw_start() requires that the descriptor ring indexes be
set to zero. Let a deferred invocation of rtl8169_reset_task()
handle it. Enabling a few power management bits will not hurt
either.

suspend/resume is issued with irq on: the spinlock do not need
to save the irq flag.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>