nfc: s3fwrn5: replace the assertion with a WARN_ON

In s3fwrn5_fw_recv_frame, if fw_info->rsp is not empty, the
current code causes a crash via BUG_ON. However, s3fwrn5_fw_send_msg
does not crash in such a scenario. The patch replaces the BUG_ON
by returning the error to the callers and frees up skb.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
Signed-off-by: David S. Miller <davem@davemloft.net>