s390/lcs: add braces around empty function body

Fix allmodconfig + W=1 compile breakage:

drivers/s390/net/lcs.c: In function ‘lcs_get_frames_cb’:
drivers/s390/net/lcs.c:1823:25: error: suggest braces around empty body in an ‘else’ statement [-Werror=empty-body]
 1823 |                         ; // FIXME: error message ?
      |                         ^

Acked-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>