[SCSI] mpt2sas: Added missing mpt2sas_base_detach call from scsih_remove context

mpt2sas_base_detach() call was removed from _scsih_remove() while
doing some code shuffling.  Mainly when we work on adding code for
scsih_shutdown().  I have added back mpt2sas_base_detach() which will
get callled from _scsih_remove().

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