mISDN: Fix wrong struct name in macro and clarifications

Based on comments from Joe Perches <joe@perches.com>. Thanks.

Fix IOFUNC_MEMIO macro. WriteFiFo##name##_MIO use the wrong struct name,
this was missed because the macro was only called with this name.

Clarify with _func that the defined types are functions.
Add names to the parameters for better understanding the purpose.

Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>