memory: atmel-ebi: use PTR_ERR_OR_ZERO() to simplify the code

Use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR.

Generated by coccinelle.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>