abis.c: Grab reference to e1inp_line_get if already created
abis.c: Grab reference to e1inp_line_get if already created

Since we are returning the pointer, it should always be grabbing a
reference (find doesn't do it).
In practice it's not much important since it is always created and not
found.

Change-Id: Ib84636663be2df33d497131c780b010b57f17e32

osmocom/osmo-bts,osmocom/osmo-bts,osmocom/osmo-bts