phy: mvebu-cp110: checking for NULL instead of IS_ERR()

devm_ioremap_resource() never returns NULL, it only returns error
pointers so this test needs to be changed.

Fixes: d0438bd6 ("phy: add the mvebu cp110 comphy driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>