ide: add ide_init_pc() helper

* Add IDE_PC_BUFFER_SIZE define.

* Add ide_init_pc() and convert ide-{floppy,tape}.c to use it
  instead of ide*_init_pc().

* Remove no longer used IDE*_PC_BUFFER_SIZE and ide*_init_pc().

There should be no functional changes caused by this patch.

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