ALSA: scarlett2: Add check for config_item presence

Update scarlett2_usb_get_config() and scarlett2_usb_set_config() to
make sure that the config_item_num is valid for the device.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/b0572b23291ffd1b208f21d298adaf4d9f1fe4bc.1703444932.git.g@b4.vu


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