bugfix: fix PDCP inst

This commit is a continuation of 614d6bbe
(ue_ip: use correct instance).

Now in openair2/NETWORK_DRIVER/UE_IP/common.c the 'inst' is not
forced to 1 anymore, we take the value 'pdcph_p->inst'.

It turns out that 'pdcph_p->inst' is 0 instead of 1 when we
run lte-softmodem as an UE.

So let's modify PDCP to set 'inst' to 1 where it was set to 0 for
the UE softmodem case, and skip the places where it is reset to 0,
still for the UE softmodem case.

This may break things, I am not sure.