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: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>