[SCSI] stex: use sg buffer copy helper functions

This replaces stex_internal_copy with scsi_sg_copy_to/from_buffer.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Ed Lin <ed.lin@promise.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>