cosmetic: it's n_r in check_for_first_ciphrd(), not n_s
cosmetic: it's n_r in check_for_first_ciphrd(), not n_s

When we introduced the n_s verification in
2cc37035d73191b71b9ba9c0d559a0da6a5f35e5, the variable name n_s was
used for what is actually n_r N(R) read from the LAPDm frame

Change-Id: Iaef1648f35ceae9d7f4cd1d9d5409e05115d199a

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