s390: qeth_core_mpc: Use ARRAY_SIZE instead of reimplementing its function

Use the common code ARRAY_SIZE macro instead of a private implementation.

Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>