samples: bluetooth: hci_rpmsg: Discard too long incoming packets

Avoid hitting the assert
```
__ASSERT_NO_MSG(net_buf_simple_tailroom(buf) >= len);
```
when the provided buffer is longer than we can fit. Instead discard the
packet and log an error.

This handling is similar to packets are discarded in hci_uart

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>