brcmfmac: use skb_cow() in brcmf_sdbrcm_txpkt() to assure alignment

In brcmf_sdbrcm_txpkt() a new packet is allocated and used to transmit
to firmware freeing up the original packet. However, that packet is
still referenced in firmware-signalling so this would result in a
double free. Using skb_cow() avoids this as the packet reference is
unchanged.

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