l1sap: fix incorrect pointer cast in l1sap_chan_act()
l1sap: fix incorrect pointer cast in l1sap_chan_act()

In [1] I introduced a regression, so osmo-bts started to complain:

  This PHY does not support lchan TSC 3 != BSIC-TSC 7

on channel activation, despite the TSC in RSL_IE_CHAN_IDENT was 7.

The problem is that this statement:

  cd = (const struct gsm48_chan_desc *) TLVP_VAL(tp, RSL_IE_CHAN_IDENT) + 1;

is basically equivalent to:

  cd = ((const struct gsm48_chan_desc *) TLVP_VAL(tp, RSL_IE_CHAN_IDENT)) + 1;

so we actually shift the pointer by sizeof(struct gsm48_chan_desc)
and skip 3 octets instead of just one (IEI octet).  Fix this.

Change-Id: Ic3a81396b60577e03c541d32839d07dc6d45c838
Fixes: [1] Id100f4c56fd5c1adad5d925d97240bed82981b9b
Fixes: OS#5121

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