brcmfmac: rework credit pickup to assure consistent handling

Reworked brcmf_skb_pick_up_credit() so it can be used for both
fcmode flavours in the same way.

Reviewed-by: Hante Meuleman <meuleman@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>