brcmfmac: Update msgbuf read pointer quicker.

On device to host data using msgbuf the read pointer gets updated
once all data is processed. Updating this pointer more frequently
allows the firmware to add more data quicker. This will result in
slightly higher and more stable throughput on CPU bounded host
processors.

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: Kalle Valo <kvalo@codeaurora.org>