ide: merge flagged_taskfile() into do_rw_taskfile()

Based on the earlier work by Tejun Heo.

task->data_phase == TASKFILE_MULTI_{IN,OUT} vs drive->mult_count == 0
check is needed also for ide_taskfile_ioctl() requests that don't have
IDE_TFLAG_FLAGGED taskfile flag set.

Cc: Tejun Heo <htejun@gmail.com>
Acked-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>