brcmfmac: fix handling sk_buff cleanup upon bus tx failure

When firmware-signalling is active the brcmf_txcomplete() does
a free of the sk_buff when transfer to firmware fails in the
bus-specific driver code. However, it should also cleanup the
packet from the hanger. This patch fixes that.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>