net/wan/fsl_ucc_hdlc: fix unitialized variable warnings

This fixes the following compiler warnings:
drivers/net/wan/fsl_ucc_hdlc.c: In function 'ucc_hdlc_poll':
warning: 'skb' may be used uninitialized in this function
[-Wmaybe-uninitialized]
  skb->mac_header = skb->data - skb->head;

and

drivers/net/wan/fsl_ucc_hdlc.c: In function 'ucc_hdlc_probe':
drivers/net/wan/fsl_ucc_hdlc.c:1127:3: warning: 'utdm' may be used
uninitialized in this function [-Wmaybe-uninitialized]
   kfree(utdm);

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Cc: Zhao Qiang <qiang.zhao@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>