media: vivid-radio-rx: add a cast to avoid a warning

The logic at vivid_radio_rx_g_tuner() is producint an overflow
warning:

	drivers/media/platform/vivid/vivid-radio-rx.c:250 vivid_radio_rx_g_tuner() warn: potential negative subtraction from max '65535 - (__builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( ==  ||  == , , __builtin_choose_expr( == , , (0))))))) * 65535) / delta'

Add a cast to prevent that.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

drivers/media/platform/vivid/vivid-radio-rx.c

@@ -247,7 +247,7 @@ int vivid_radio_rx_g_tuner(struct file *file, void *fh, struct v4l2_tuner *vt)
 	vt->rangehigh = FM_FREQ_RANGE_HIGH;
 	sig_qual = dev->radio_rx_sig_qual;
 	vt->signal = abs(sig_qual) > delta ? 0 :
-		     0xffff - (abs(sig_qual) * 0xffff) / delta;
+		     0xffff - ((unsigned)abs(sig_qual) * 0xffff) / delta;
 	vt->afc = sig_qual > delta ? 0 : sig_qual;
 	if (abs(sig_qual) > delta)
 		vt->rxsubchans = 0;