l1sap: Correctly count RACH slots in calc_exprd_rach_frames()

We used a bogus multiplication factor of four when computing the number
of expired RACH slots.  While there are four RACH slots per block (i.e.
4 times more RACH received than normal MAC blocks), that multiplier
doesn't apply here:  We are calling this function per *frame* and not
per *block*.  So the maximum number of RACH slots per *frame* is (in
most suual cases with a single CCCH) at maximum 1. Only some obscure
configurations with multiple CCCHs in a single cell would render higher
values.  In any case, *blocks* never even enter this equation.

This wrong multiplier resulted in rather weird RACH load reports to the
BSC.

Related: OS#3750
Change-Id: I6b14fd6e7819f9164fb4a09b432a9f419e3b6e5c

src/common/l1sap.c

@@ -484,10 +484,7 @@ static unsigned int calc_exprd_rach_frames(struct gsm_bts *bts, uint32_t fn)
 			rach_frames_expired = 1;
 	}
 
-	/* Each Frame has room for 4 RACH slots, since RACH
-	 * slots are short enough to fit into a single radio
-	 * burst, so we need to multiply the final result by 4 */
-	return rach_frames_expired * 4;
+	return rach_frames_expired;
 }
 
 /* time information received from bts model */