[SCSI] mpt2sas: add missing initialization of scsih_cmds

Internal command scsih_cmds init is included in mpt2sas_base_attach.

Signed-off-by: Kashyap Desai <kashyap.desai@lsi.com>
Cc: Stable Tree <stable@kernel.org>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>