brcmfmac: Take bus flowcontrol at credit mgmt into account.

On bus flow control (no more host bus resources to send packets
to device) the netif flow control was toggled, however credit
management should also take this status into account. Since there
are multiple sources handling this flow control necessary spinlocks
were added to protect flow control related data/states.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>