[SCSI] mpt2sas: add missing allocation.

There was supposed to be a kzalloc() here and the compiler complained
about it.
mpt2sas_scsih.c: In function ‘mpt2sas_scsih_reset_handler’:
mpt2sas_scsih.c:2807:21: warning: ‘fw_event’ may be used uninitialized in this function [-Wuninitialized]

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: "Nandigama, Nagalakshmi" <Nagalakshmi.Nandigama@lsi.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>