helpers/labgrid-raw-interface: make tcpdump count argument actually optional

The intention of labgrid-raw-interface was to have an optional "count"
argument, allowing for packet capture on an interface until a timeout is
reached, no matter how many packets are received. This is used when
calling:

  RawNetworkInterfaceDriver.start_record(filename, timeout=10)

However, the missing `nargs="?"` kwarg when adding the argument actually
required the argument to be given:

  usage: labgrid-raw-interface tcpdump [-h] [--timeout TIMEOUT] ifname count
  labgrid-raw-interface tcpdump: error: the following arguments are
  required: count

The original commit introducing the option actually included it [1]. Fix
that by re-adding `nargs="?"`.

[1] 90cf941 ("helpers: add labgrid-raw-interface helper")

Fixes: c1aec37 ("helpers/labgrid-raw-interface: introduce subparsers per program, pass options")
Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

helpers/labgrid-raw-interface

@@ -126,7 +126,7 @@ if __name__ == "__main__":
     # tcpdump
     tcpdump_parser = subparsers.add_parser("tcpdump")
     tcpdump_parser.add_argument("ifname", type=str, help="interface name")
-    tcpdump_parser.add_argument("count", type=int, default=None, help="amount of frames to capture while recording")
+    tcpdump_parser.add_argument("count", type=int, nargs="?", default=None, help="amount of frames to capture while recording")
     tcpdump_parser.add_argument(
         "--timeout", type=int, default=None, help="Amount of time to capture while recording. 0 means capture forever"
     )