Bluetooth: Host: Check returned value by `LE_READ_BUFFER_SIZE`

`rp->le_max_num` was passed unchecked into `k_sem_init()`, this could
lead to the value being uninitialized and an unknown behavior.

To fix that issue, the `rp->le_max_num` value is checked the same way as
`bt_dev.le.acl_mtu` was already checked. The same things has been done
for `rp->acl_max_num` and `rp->iso_max_num` in
`read_buffer_size_v2_complete()` function.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>