ALSA: core: Add snd_device_get_state() helper

A new small helper to get the current state of the device registration
for the given object.  It'll be used for USB-audio driver to check the
delayed device registrations.

Link: https://lore.kernel.org/r/20200323170643.19181-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>