ARM: s3c64xx: fix return value check in s3c_usb_otgphy_init()

The function clk_get() returns ERR_PTR() in case of error and
never returns NULL. So there's no need to test whether xusbxti
is NULL, just remove the redundant part in the return value check.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>