mpt3sas: combine fw_event_work and its event_data

Tack the firmware reply event_data payload to the end of its
corresponding struct fw_event_work allocation.  This matches the
convention in the mptfusion driver and simplifies the code.

This avoids the following smatch warning:

  drivers/scsi/mpt3sas/mpt3sas_scsih.c:2519
    mpt3sas_send_trigger_data_event() warn: possible memory leak of
    'fw_event'

Signed-off-by: Joe Lawrence <joe.lawrence@stratus.com>
Acked-by: Sreekanth Reddy <Sreekanth.Reddy@avagotech.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>