phy: qualcomm: fix return value check in qcom_ipq806x_usb_phy_probe()

phy: qualcomm: fix return value check in qcom_ipq806x_usb_phy_probe()

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

Fixes: ef19b117b834 ("phy: qualcomm: add qcom ipq806x dwc usb phy driver")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20200723113622.136752-1-weiyongjun1@huawei.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>