i2c: designware: Fix return value check in navi_amd_register_client()

In case of error, the function i2c_new_client_device() returns
ERR_PTR() and never returns NULL. The NULL test in the return
value check should be replaced with IS_ERR().

Fixes: 17631e8c

 ("i2c: designware: Add driver support for AMD NAVI GPU")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>