Bluetooth: Remove pointless parameter check in btmrvl_send_frame()

The hdev parameter of btmrvl_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>