s390x/event-facility: fix location of receive mask

s390x/event-facility: fix location of receive mask

For read event mask, we assumed that the layout of the sccb was

|sccb header|event buffer header|receive mask|...|

The correct layout, however, is

|sccb header|receive mask|...|

as in-buffer and

|sccb header|event buffer header|...|

as out-buffer.

Fix this: This makes selective read work.

Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>