shell: telnet: Fix handling of multiple commands in a single packet
shell: telnet: Fix handling of multiple commands in a single packet

In case multiple commands were sent in a single packet, only the first
one was processed. Fix this by processing them in a loop.

As the subnegotiation command have variable length, and we currently
don't have any support for subnegotiation, stop processing packet when
such a command occurs.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr