Bluetooth: hci_uart: Fix dereferencing of ERR_PTR

If h4_recv_buf() return ERR_PTR instead sk_buff pointer, it should be
cleared once PTR_ERR is completed for the further dereference such as
h4_recv(), or h4_close().

Signed-off-by: Chan-yeol Park <chanyeol.park@samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>