broadband-modem,cli: slightly bump 3GPP scan timeout; add some cli dbus timeout slack

Add some mmcli D-Bus timeout slack, making them slightly higher than the
ModemManager internal timeout, to ensure the cli doesn't time out the
D-Bus call when MM succeeds.

For 3GPP scan, some modems appear to have internal 5 minute timeouts,
so ensure MM doesn't time out right before the modem returns results:

ModemManager[282578]: <dbg> [1741054422.321400] [ttyUSB2/at] --> 'AT+COPS=?<CR>'
ModemManager[282578]: <wrn> [1741054722.247806] [modem1] failed scanning networks: Serial command timed out
ModemManager[282578]: <dbg> [1741054724.119770] [ttyUSB2/at] <-- '<CR><LF>+COPS: (1,"T-Mobile","T-Mobile","310260",0),,(0,1,2,3,4),(0,1,2)<CR><LF><CR><LF>OK<CR><LF>'

Signed-off-by: Dan Williams <dan@ioncontrol.co>

src/mm-broadband-modem.c

@@ -5317,7 +5317,7 @@ modem_3gpp_scan_networks (MMIfaceModem3gpp *self,
 {
     mm_base_modem_at_command (MM_BASE_MODEM (self),
                               "+COPS=?",
-                              300,
+                              315,
                               FALSE,
                               callback,
                               user_data);