ms: Add current_pacch_slots method

The PACCH is specific to an MS and may change when a TBF is
established or removed (see TS 44.060, 8.1.1.2.2). For multislot
class type 2 phones, more than one timeslot may be used to transmit
RLC/MAC control messages.

Add a method that returns a set of TS which can be used for the
PACCH.

Sponsored-by: On-Waves ehf
diff --git a/src/gprs_ms.h b/src/gprs_ms.h
index 246f71e..b07f175 100644
--- a/src/gprs_ms.h
+++ b/src/gprs_ms.h
@@ -97,6 +97,7 @@
 	uint8_t ul_slots() const;
 	uint8_t reserved_dl_slots() const;
 	uint8_t reserved_ul_slots() const;
+	uint8_t current_pacch_slots() const;
 	gprs_rlcmac_trx *current_trx() const;
 	void set_reserved_slots(gprs_rlcmac_trx *trx,
 		uint8_t ul_slots, uint8_t dl_slots);