cxl/core: Fix initialization of mbox_cmd.size_out in get event

cxl/core: Fix initialization of mbox_cmd.size_out in get event

Since mbox_cmd.size_out is overwritten with the actual output size in
the function below, it needs to be initialized every time.

cxl_internal_send_cmd -> __cxl_pci_mbox_send_cmd

Problem scenario:

1) The size_out variable is initially set to the size of the mailbox.
2) Read an event.
   - size_out is set to 160 bytes(header 32B + one event 128B).
   - Two event are created while reading.
3) Read the new *two* events.
   - size_out is still set to 160 bytes.
   - Although the value of out_len is 288 bytes, only 160 bytes are
     copied from the mailbox register to the local variable.
   - record_count is set to 2.
   - Accessing records[1] will result in reading incorrect data.

Fixes: 6ebe28f9ec72 ("cxl/mem: Read, trace, and clear events on driver load")
Tested-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Kwangjin Ko <kwangjin.ko@sk.com>
Signed-off-by: Dave Jiang <dave.jiang@intel.com>