pktgen: simplify error handling in pgctrl_write()

The 'out' label is just a relict from previous times as pgctrl_write()
had multiple error paths. Get rid of it and simply return right away
on errors.

Cc: "David S. Miller" <davem@davemloft.net>
Signed-off-by: Mathias Krause <minipli@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>