ALSA: usb-audio: Fix missing braces in some struct inits

The struct s1810c_state_packet contains the array in the first field
hence zero-initialization requires a more couple of braces.  Fix the
compile warning pointing it out:
   sound/usb/mixer_s1810c.c: In function 'snd_sc1810c_get_status_field':
   sound/usb/mixer_s1810c.c:178:9: warning: missing braces around initializer [-Wmissing-braces]

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 8dc5efe3 ("ALSA: usb-audio: Add support for Presonus Studio 1810c")
Link: https://lore.kernel.org/r/202002210251.WgMfvKJP%lkp@intel.com
Link: https://lore.kernel.org/r/20200306081231.7940-1-tiwai@suse.de


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