Changes

trau_frame encode8_hr: simplify setting of C5 for odd parity

As detailed in the previous commit, the API "contract" of
osmo_trau_frame_encode() for OSMO_TRAU8_SPEECH in DL direction
immortalizes a strange choice where C4 is controlled by the user
while C1..C3 and C5 are overridden by the function.  C5 is the
odd parity bit; because C1..C3 are fixed and only C4 comes from
the user, the correct odd parity bit C5 can be set more efficiently
by simply inverting C4, skipping full computation of OP.

Change-Id: I30c7dfdaaadd0fd4cb084cf02c66c0f19a40ae42