yam: integer underflow in yam_ioctl()

We cap bitrate at YAM_MAXBITRATE in yam_ioctl(), but it could also be
negative.  I don't know the impact of using a negative bitrate but let's
prevent it.
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/linux/yam.h

@@ -77,6 +77,6 @@ struct yamdrv_ioctl_cfg {
 
 struct yamdrv_ioctl_mcs {
 	int cmd;
-	int bitrate;
+	unsigned int bitrate;
 	unsigned char bits[YAM_FPGA_SIZE];
 };