SI13: Set alpha value to a safer default of 0 instead of 10

alpha=0 (the new value) doesn't reduce MS transmission power during GPRS
as much as we did with alpha=10.  This is to optimize for coverage and
to keep GPRS working at all cost, and not care about MS battery life
time or uplink interference in surrounding cells.

FIXME: This should be made configurable via the VTY and the normal
default (unless configured otherwise by vty/config file) should be '6'.

openbsc/src/libbsc/system_information.c

@@ -634,7 +634,7 @@ static struct gsm48_si13_info si13_default = {
 		},
 	},
 	.pwr_ctrl_pars = {
-		.alpha		= 10,	/* a = 1.0 */
+		.alpha		= 0,	/* a = 0.0 */
 		.t_avg_w	= 16,
 		.t_avg_t	= 16,
 		.pc_meas_chan	= 0, 	/* downling measured on CCCH */