Since the commit 50bed2e2862a8f3a4f7d683d0d27292e71ef18b9 (sg: disable
interrupts inside sg_copy_buffer), no need to disable interrupts
before calling scsi_sg_copy_from_buffer. So we can simplify
twa_scsiop_execute_scsi_complete() a bit, which disables interrupts
just for scsi_sg_copy_from_buffer.