ide: remove 'handler' field from ide_task_t (take 2)

ide: remove 'handler' field from ide_task_t (take 2)

* Add IDE_TFLAG_CUSTOM_HANDLER taskfile flag and use it for internal requests
  which require custom handlers.  Check the flag in do_rw_taskfile() and set
  handler accordingly.

* Cleanup ide_init_{specify,restore,setmult}_cmd() and rename it to
  ide_tf_set_{specify,restore,setmult}_cmd().

* Make {set_geometry,recal,set_multmode}_intr() static.

* Remove no longer needed 'handler' field from ide_task_t.

v2:
* 'handler' in do_rw_taskfile() must be set to NULL initially.

There should be no functionality changes caused by this patch.

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