Move idescsi_map_sg() call out from idescsi_issue_pc()
to idescsi_do_request() as a preparation to adding generic
ide_issue_pc() helper.

There should be no functional changes caused by this patch.

Cc: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>