brcmfmac: Improve throughput by scheduling msbug flow worker.

The tx flow worker in msgbuf gets scheduled at tx till a certain
threshold has been reached. Then the tx completes will take over
the scheduling. When amsdu and ampdu is used the frames are
transferred wireless in a very bulky fashion, in combination
with this scheduling algorithm and buffer limiters in the stack
this can result in limited throughput. This change causes the
flow worker to be scheduled more frequently from tx.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@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>