powerpc/mv643xx_eth: fix return check in mv64x60_eth_register_shared_pdev()

In case of error, the function platform_device_register_simple() returns
RR_PTR() and never returns NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Scott Wood <scottwood@freescale.com>