Bluetooth: btnxpuart: remove useless assignment

Remove useless assignment of rx_skb to NULL in case the skb is in error,
this is already done in h4_recv_buf() that is executed a few lines
before.

Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>