Since commit 9cc4b7cb ("brcmfmac: Make skb header writable
before use") the headroom usage has been fixed. However, the
driver was keeping statistics that got lost. So reworking the
code so we get those driver statistics back for debugging.

Cc: James Hughes <james.hughes@raspberrypi.org>
Reviewed-by: NHante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: NPieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: NFranky Lin <franky.lin@broadcom.com>
Signed-off-by: NArend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: NKalle Valo <kvalo@codeaurora.org>