brcmfmac: rework headroom check in .start_xmit()

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: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>