ide-tape: simplify code branching in the interrupt handler

... by adding a new typedef function pointer idetape_io_buf in order to call
the proper buffer i/o handler depending on the data direction.

Bart:
- move idetape_io_buf before idetape_pc_intr() comment

Signed-off-by: Borislav Petkov <petkovbb@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>