ata: libata-core: make ata_exec_internal_sg() *static*

ata_exec_internal_sg() is only called by ata_exec_internal() further in
the same file, so we can make it *static* and remove its prototype from
drivers/ata/libata.h...

Suggested-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>