ata: libata-sff: kill unused ata_sff_busy_sleep()

Nobody seems to call ata_sff_busy_sleep(), so we can get rid of it...

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