brcmfmac: explicitly indicate sk_buff is sent upon request credit

Firmware can request the driver for transmit packets using two different
signals. Only for one signal a flag was set in the sk_buff control
buffer. This patch adds explicit flag for the other signal as well.

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>