phy: qcom-qmp: fix return value check in qcom_qmp_phy_create()

In case of error, the function of_iomap() returns NULL pointer
not ERR_PTR(). The IS_ERR() test in the return value check should
be replaced with NULL test.

Fixes: e78f3d15

 ("phy: qcom-qmp: new qmp phy driver for qcom-chipsets")
Reviewed-by: Vivek Gautam <vivek.gautam@codeaurora.org>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>