Bluetooth: shell: Fix implicit gatt client requirement for bt shell

`write_stats` functionality implementation is guarded by
`BT_GATT_CLIENT`, but is also called from commands defined when
`BT_GATT_DYNAMIC_DB` is set. This means that now application will not
build with `BT_SHELL` used, when we set `BT_GATT_DYNAMIC_CB` solely.

Move this functionality to top of the file without guarding
ifdefs, so it can be used in all gatt shell commands.

Signed-off-by: Marek Metelski <marek.metelski01@gmail.com>