Improve the one-shot ELD repoll to up to 6 retries.

Up to now the 300ms looks sufficient for the test boxes. However
I'm a bit worried about how well it can fit the wider user base.
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>