ocxl: Fix missing unlock on error in afu_ioctl_enable_p9_wait()

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: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Alastair D'Silva <alastair@d-silva.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>