soc: ti: omap-prm: fix return value check in omap_prm_probe()

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

Fixes: 3e99cb21 ("soc: ti: add initial PRM driver with reset control support")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>