Bluetooth: hci_ll: Fix NULL pointer deref on FW upload failure

Avoid NULL pointer dereference occurring due to freeing
skb containing an error pointer. It can easily be triggered
by using the driver with broken uart (i.e. due to misconfigured
pinmuxing).

Fixes: 37180552 ("bluetooth: hci_uart: add LL protocol serdev driver support")
Signed-off-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>