wlcore: add a debugfs entry to allow changing the sleep mode by hand

For FW debugging purposes, we may need to change the sleep mode
(aka. sleep_auth) by hand, and set it to the mode we want.  To allow
this, a debugfs entry is added.

Now we store the sleep_auth value that has been set and use that
instead of the quirk to decide whether we should enter ELP or not.

Signed-off-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>