[SCSI] 3w-xxxx: use sg buffer copy helper functions

This rewrites tw_transfer_internal with scsi_sg_copy_from/to_buffer.

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