Bluetooth: Shell: Fix GATT exchange mtu assertion fail

This change fixes a bug where the second use of shell command `gatt
exchange-mtu` would trigger an assertion failure in the stack. The BT
API `bt_gatt_exchange_mtu` asserts `params->func != NULL`.

Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>