memory: atmel-ebi: fix return value check in at91_ebi_dev_disable()

Fix the retrn value check which testing the wrong variable
in at91_ebi_dev_disable().

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>