powerpc-opal: fix spelling mistake "Uniterrupted" -> "Uninterrupted"

Trivial fix to spelling mistake in hmi_error_types text

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Stewart Smith <stewart@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>