Add the missing unlock before return from function
afu_ioctl_enable_p9_wait() in the error handling case.

Fixes: e948e06f ("ocxl: Expose the thread_id needed for wait on POWER9")
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: NAlastair D'Silva <alastair@d-silva.org>
Signed-off-by: NMichael Ellerman <mpe@ellerman.id.au>