l1sap: Pass is_sub from L1 primitive into the Uplink Measurement
l1sap: Pass is_sub from L1 primitive into the Uplink Measurement

info_meas_ind on the L1SAP always allowed the lower layers to pass
in whether a given measurement is part of the "SUB", or not.

However, the existing l1sap code before this patch simply drops this
information, despite the measurement.c code also having "is_sub" state.

Let's make sure this state is passed from L1SAP into measurement
processing as intended.

Fact is, none of our current lower-layers actually set this is_sub flag
for their primitives passed up in L1SAP, but at least now *if* they
would set that flag, the measurement code would process it as intended.

Related: OS#2978
Change-Id: Ibed2e8d7563b471c6b5dd2214ac4765caf31ed2a

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