server/telnet: simplify telnet_input function

server/telnet: simplify telnet_input function

running complexity on this file tells that:
NOTE: proc telnet_input in file telnet_server.c line 576
	nesting depth reached level 8
==>	*seriously consider rewriting the procedure*.
Complexity Scores
Score | ln-ct | nc-lns| file-name(line): proc-name
  319     272     226   src/server/telnet_server.c(576): telnet_input
total nc-lns      226

so try to reduce the complexity score of telnet_input function

Change-Id: I64ecb0c54da83c27a343f2a1df99fc8f9484572a
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6440
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>