usb: don't use 'Yoda conditions'

imitate nearby code about using '!value' or 'value == NULL'
Signed-off-by: NGonglei <arei.gonglei@huawei.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

hw/usb/dev-audio.c

@@ -371,7 +371,7 @@ static void output_callback(void *opaque, int avail)
             return;
         }
         data = streambuf_get(&s->out.buf);
-        if (NULL == data) {
+        if (!data) {
             return;
         }
         AUD_write(s->out.voice, data, USBAUDIO_PACKET_SIZE);

hw/usb/dev-mtp.c

@@ -832,7 +832,7 @@ static void usb_mtp_command(MTPState *s, MTPControl *c)
             return;
         }
         data_in = usb_mtp_get_object(s, c, o);
-        if (NULL == data_in) {
+        if (data_in == NULL) {
             usb_mtp_queue_result(s, RES_GENERAL_ERROR,
                                  c->trans, 0, 0, 0);
             return;
@@ -851,7 +851,7 @@ static void usb_mtp_command(MTPState *s, MTPControl *c)
             return;
         }
         data_in = usb_mtp_get_partial_object(s, c, o);
-        if (NULL == data_in) {
+        if (data_in == NULL) {
             usb_mtp_queue_result(s, RES_GENERAL_ERROR,
                                  c->trans, 0, 0, 0);
             return;

hw/usb/hcd-ehci.c

@@ -1596,7 +1596,7 @@ static EHCIQueue *ehci_state_fetchqh(EHCIState *ehci, int async)
 
     entry = ehci_get_fetch_addr(ehci, async);
     q = ehci_find_queue_by_qh(ehci, entry, async);
-    if (NULL == q) {
+    if (q == NULL) {
         q = ehci_alloc_queue(ehci, entry, async);
     }