[SCSI] gdth: split out eisa probing

Split eisa probing into it's own helper, and do proper error unwinding.
Protect EISA probind by the proper CONFIG_EISA symbol.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>