[SCSI] sun3_NCR5380: Split NEXT() for lvalues/rvalues

Using the current NEXT() macro for both lvalues and rvalues gives:

In file included from drivers/scsi/sun3_scsi.c:623:
drivers/scsi/sun3_NCR5380.c: In function ‘NCR5380_queue_command_lck’:
drivers/scsi/sun3_NCR5380.c:993: warning: assignment discards qualifiers from pointer target type
drivers/scsi/sun3_NCR5380.c: In function ‘NCR5380_main’:
drivers/scsi/sun3_NCR5380.c:1147: warning: assignment discards qualifiers from pointer target type
drivers/scsi/sun3_NCR5380.c: In function ‘NCR5380_information_transfer’:
drivers/scsi/sun3_NCR5380.c:2277: warning: assignment discards qualifiers from pointer target type
drivers/scsi/sun3_NCR5380.c:2333: warning: assignment discards qualifiers from pointer target type

Change NEXT() to operate on rvalues only(), and introduce SET_NEXT() to
operate on lvalues, as is done in drivers/scsi/atari_NCR5380.c.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>