s390: Fix possibly wrong size in strncmp (smsgiucv)

This error was reported by cppcheck:
drivers/s390/net/smsgiucv.c:63: error: Using sizeof for array given as
function argument returns the size of pointer.

Although there is no runtime problem as long as sizeof(u8 *) == 8,
this misleading code should get fixed.

Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>