Bluetooth: L2CAP: Add check to see if device is not disconnected

Currently, if a bluetooth peripheral sends a connection parameter update
request to a Zephyr central while the central is in the process of
disconnecting and has sent a connection terminate command in the same
connection interval, the central will try to reply, resulting in an
error being printed in the console.

This commit adds a check in le_conn_param_update_req that catches this
state before the central considers this request, preventing the unneeded
error being printed.

Fixes #48813

Signed-off-by: Ivan Herrera Olivares <ivan.herreraolivares@uantwerpen.be>