V4L/DVB: firedtv: reduce memset()s

Before each FCP transdaction, the entire 512 bytes of the FCP frame were
cleared, then values filled in.

Clear only the bytes between filled-in bytes and end of the
  - request frame, or
  - response frame if data from a larger response will be needed, or
  - whole frame if data from a variable length response will be taken.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>