net: thunderx: avoid dereferencing xcv when NULL

This fixes the following smatch and coccinelle warnings:

  drivers/net/ethernet/cavium/thunder/thunder_xcv.c:119 xcv_setup_link() error: we previously assumed 'xcv' could be null (see line 118) [smatch]
  drivers/net/ethernet/cavium/thunder/thunder_xcv.c:119:16-20: ERROR: xcv is NULL but dereferenced. [coccinelle]

Fixes: 6465859aba1e66a5 ("net: thunderx: Add RGMII interface type support")
Signed-off-by: Vincent Stehlé <vincent.stehle@laposte.net>
Cc: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>