[SCSI] mpt2sas: Null pointer deference possibility in mpt2sas_ctl_event_callback function

Added a check to identify if mpi_reply is NULL in mpt2sas_ctl_event_callback()
and return without proceeding if it is the case.

Also modified the following functions to return void instead of 0 or 1
as returning those values from events perspective doesn't make sense.
* _base_async_event()
* mpt2sas_ctl_event_callback()
* mpt2sas_scsih_event_callback()

Signed-off-by: Sreekanth Reddy <Sreekanth.Reddy@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>