net: pktgen: fix ctrl interface command parsing

Enable command writing without trailing '\n':

- the good case

	$ echo "reset" > /proc/net/pktgen/pgctrl

- the bad case (before the patch)

	$ echo -n "reset" > /proc/net/pktgen/pgctrl
	-bash: echo: write error: Invalid argument

- with patch applied

	$ echo -n "reset" > /proc/net/pktgen/pgctrl

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250219084527.20488-7-ps.report@gmx.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>