Increase SNMP timeout to 2500 ms in configuration file.

Value of 1500 ms is too small to detect devices in deep sleep state.
This change make timeout consistent between config file and source code,
defined in backend/magicolor.c:

static int MC_SNMP_Timeout = 2500;

Configuration value always takes higher precedence.

backend/magicolor.conf.in

@@ -6,8 +6,8 @@
 ###                   read requests. All values are given in milliseconds,
 ###                   i.e. 1000 is 1 second.
 
-# SNMP auto-detection waits 1.5 seconds
-snmp-timeout 1500
+# SNMP auto-detection waits 2.5 seconds
+snmp-timeout 2500
 
 # wait 15 seconds for scan data (color scans take ~6 seconds to initialize,
 # so we need to wait longer than that)