shell: backend: telnet: Don't assert if connection closed

The code in shell_ops.c that calls telnet_write will assert if it
returns non-zero. For a telnet shell it's normal that the
network might disconnect unexepectedly, so that should not
trigger an assert.

Fixes #67637

Link: https://github.com/zephyrproject-rtos/zephyr/issues/67637



Signed-off-by: Kevin ORourke <kevin.orourke@ferroamp.se>