HSI: ssi_protocol: double free in ssip_pn_xmit()

If skb_pad() fails then it frees skb and we don't need to free it again
at the end of the function.

Fixes: dc7bf5d7 ("HSI: Introduce driver for SSI Protocol")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sebastian Reichel <sre@kernel.org>