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

sound/usb/mixer_s1810c.c

@@ -175,8 +175,8 @@ static int
 snd_sc1810c_get_status_field(struct usb_device *dev,
 			     u32 *field, int field_idx, uint16_t *seqnum)
 {
-	struct s1810c_state_packet pkt_out = { 0 };
-	struct s1810c_state_packet pkt_in = { 0 };
+	struct s1810c_state_packet pkt_out = { { 0 } };
+	struct s1810c_state_packet pkt_in = { { 0 } };
 	int ret = 0;
 
 	pkt_out.fields[SC1810C_STATE_F1_IDX] = SC1810C_SET_STATE_F1;