[media] gscpa: get rid of warning: suggest braces around empty body

There are hundreds of messages like this one, when GSPCA debug is
disabled, when compiled with W=1:
drivers/media/usb/gspca/spca500.c:725:46: warning: suggest braces around empty body in an 'if' statement [-Wempty-body]
Get rid of it, especially as it might actually cause troubles on
some places.

Cc: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/media/usb/gspca/gspca.h

@@ -32,7 +32,7 @@ do {								\
 #define D_USBO 0x00
 #define D_V4L2 0x0100
 #else
-#define PDEBUG(level, fmt, ...)
+#define PDEBUG(level, fmt, ...) do {} while(0)
 #endif
 
 #define GSPCA_MAX_FRAMES 16	/* maximum number of video frame buffers */