libata: Make ata_sg_clean() static again

Commit 70e6ad0c ("[PATCH] libata: prepare ata_sg_clean() for
invocation from EH") made ata_sg_clean() global, but no user outside
libata-core.c has ever materialized.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Tejun Heo <tj@kernel.org>