firewire: Remove function callback casts

In 1394 OHCI specification, Isochronous Receive DMA context has several
modes. One of mode is 'BufferFill' and Linux FireWire stack uses it to
receive isochronous packets for multiple isochronous channel as
FW_ISO_CONTEXT_RECEIVE_MULTICHANNEL.

The mode is not used by in-kernel driver, while it's available for
userspace. The character device driver in firewire-core includes
cast of function callback for the mode since the type of callback
function is different from the other modes. The case is inconvenient
to effort of Control Flow Integrity builds due to
-Wcast-function-type warning.

This commit removes the cast. A static helper function is newly added
to initialize isochronous context for the mode. The helper function
arranges isochronous context to assign specific callback function
after call of existent kernel API. It's noticeable that the number of
isochronous channel, speed, and the size of header are not required for
the mode. The helper function is used for the mode by character device
driver instead of direct call of existent kernel API.

The same goal can be achieved (in the ioctl_create_iso_context function)
without this helper function as follows:
- Call the fw_iso_context_create function passing NULL to the callback
  parameter.
- Then setting the context->callback.sc or context->callback.mc
  variables based on the a->type value.

However using the helper function created in this patch makes code more
clear and declarative. This way avoid the call to a function with one
purpose to achieved another one.

Co-developed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Co-developed-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Oscar Carter <oscar.carter@gmx.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Testeb-by: Takashi <Sakamoto&lt;o-takashi@sakamocchi.jp>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>