Bluetooth: btusb: Remove redundant call to btusb_free_frags()

The btusb_disconnect() callback calls hci_unregister_dev() which in turn
calls btusb_close() if the HCI device is powered. The btusb_close()
function in turn will call btusb_free_frags(). It's therefore
unnecessary to have another call to btusb_free_frags() in the
btusb_disconnect() function. Besides the redundancy the second call
seems to also cause some strange stability issues which this patch then
also fixes.

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