bts: Remember the last AMR mode that we received

Not every air message contains the AMR mode so we need to remember
it to not confuse receiving equipment like AudioCodes Media Gateways.

openbsc/include/openbsc/gsm_data_shared.h

@@ -291,6 +291,7 @@ struct gsm_lchan {
 			uint8_t buf[16];
 			uint8_t len;
 		} last_sid;
+		uint8_t last_cmr;
 	} tch;
 	/* BTS-side ciphering state (rx only, bi-directional, ...) */
 	uint8_t ciph_state;