firewire: core: change type of a data buffer

from array of char to union of structs.  I already used a union to size
the buffer which holds ioctl arguments; more consequent is to define it
as an instance of this union in the first place.

Also rename several local variables from "request" to "a"(rgument) since
the term request can be mistaken to mean a transaction subaction, e.g.
an instance of struct fw_request.
Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>