Changes

osmo_trau_frame_encode(): fix encoding of 16k O&M frames

The behavior of osmo_trau_frame_encode() for OSMO_TRAU16_FT_OAM
has the same unfortunate (but now officially documented) design
quirk as all other 16k frame types: bits C1..C5 are set internally
by the encoding function and not from caller-supplied fr->c_bits[].
However, there was also a bug: bits C6..C15, which are taken from
fr->c_bits[] for all frame types, were copied from the wrong part
of this fr->c_bits[] array.  Fix the latter bug.

Change-Id: Ia366d5b0385b764c492bbed9a030ca27db71fcff