net: wwan: iosm: consolidate trace port init code

Move the channel related structures initialization from
ipc_imem_channel_init() to ipc_trace_init() and call it directly. On the
one hand, this makes the trace port initialization symmetric to the
deitialization, that is, it removes the additional wrapper.

On the other hand, this change consolidates the trace port related code
into a single source file, what facilitates an upcoming disabling of
this functionality by a user choise.

Signed-off-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Acked-by: M Chetan Kumar <m.chetan.kumar@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>