Currently line6 hwdep interface ignores O_NONBLOCK flag when
opening device and it renders it somewhat useless when using poll.

Check for O_NONBLOCK flag when opening device and don't block read()
if it is set.
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Link: https://lore.kernel.org/r/20200502193120.79115-2-anarsoul@gmail.comSigned-off-by: Takashi Iwai <tiwai@suse.de>