brcmfmac: Fix double freeing in the fmac usb data path

When the brcmf_fws_process_skb() fails to get hanger slot for
queuing the skb, it tries to free the skb.
But the caller brcmf_netdev_start_xmit() of that funciton frees
the packet on error return value.
This causes the double freeing and which caused the kernel crash.

Signed-off-by: Raveendran Somu <raveendran.somu@cypress.com>
Signed-off-by: Chi-hsien Lin <chi-hsien.lin@cypress.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1585124429-97371-3-git-send-email-chi-hsien.lin@cypress.com