Bluetooth: Remove pointless parameter check in btuart_hci_send_frame()

The hdev parameter of btuart_hci_send_frame() is always valid. If it
were not valid, then it would have crashed earlier in the call chain.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>