ata: fix return value check in ahci_seattle_get_port_info()

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

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Tejun Heo <tj@kernel.org>