clk: msm: msm-clock-controller: check for null along with error

While parsing the device tree node, perform a null check
along with the error check by substituting IS_ERR with
IS_ERR_OR_NULL on the parser pointer.

CRs-Fixed: 971705
Change-Id: Id91981be0cdc135099b8e3d9866549268ab237a3
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>