jazz_led: fix bad snprintf

Detected by GCC 7's -Wformat-truncation.  snprintf writes at most
2 bytes here including the terminating NUL, so the result is
truncated.  In addition, the newline at the end is pointless.
Fix the buffer size and the format string.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
