[SCSI] mpt2sas: Added -ENOMEM return type when allocation fails

In the driver mpt2sas_base_attach subroutine, we need to add
support to return the proper error code when there are memory allocation
failures, e.g. returning -ENOMEM.

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