brcmsmac: AP mode: network stalls frequently for power-saving stations

[Ali MJ An-Nasrawy <alimjalnasrawy@gmail.com>]

Hi

I have been investigating a network problem that I have when connecting
my phones to my laptop as an access point. The problem manifests as
highly variable ping latencies (1ms to several seconds from AP to
clients specifically) and frequent network stalls at random occasions
rendering it unusable for certain applications.

After investigation, I have found that it was due to incomplete
implementation of AP mode for brcmsmac regarding power management:
1. TIM is not updated for beacons as needed and, thus, power-saving
stations have no way to know about buffered traffic for them until they
wake up for different reason.
2. Transmission of broadcast packets doesn't obey DTIM period: they are
transmitted at any time.

I was able to fix point 1 with probably inefficient and buggy patch
that I shall submit and have no idea regarding point 2.

Hope to see professional fixes,
Thank you.