[SCSI] 3w-9xxx: remove unnecessary local_irq_save/restore for scsi sg copy API

Since the commit 50bed2e2

 (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.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Acked-by: Adam Radford <linuxraid@amcc.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>