[SCSI] mpt2sas: Introduced check for enclosure_handle to avoid crash

Kernel panic is seen because of enclosure_handle received from FW is zero.
Check is introduced before calling mpt2sas_config_get_enclosure_pg0.

Signed-off-by: Kashyap Desai <kashyap.desai@lsi.com>
Signed-off-by: James Bottomley <James.Bottomley@suse.de>