[media] radio-si476x: add return value check to avoid dead code

Dead code found on below code:
    si476x_radio_add_new_custom(radio, SI476X_IDX_DIVERSITY_MODE);
    if (rval < 0)
            goto exit;

    si476x_radio_add_new_custom(radio, SI476X_IDX_INTERCHIP_LINK);
    if (rval < 0)  ====> Dead code !!!
            goto exit;

The piece of code miss return value check after calling .si476x_radio_add_new_custom(),
the patch fix it.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>