s390/test_unwind: fix spelling mistake "reqister" -> "register"

There is a spelling mistake in a pr_info message. Fix it.

Link: https://lkml.kernel.org/r/20191202090215.28766-1-colin.king@canonical.com


Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>