ALSA: scarlett2: Add missing mutex lock around get meter levels

As scarlett2_meter_ctl_get() uses meter_level_map[], the data_mutex
should be locked while accessing it.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Fixes: 3473185f ("ALSA: scarlett2: Remap Level Meter values")
Link: https://lore.kernel.org/r/77e093c27402c83d0730681448fa4f57583349dd.1703001053.git.g@b4.vu


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