staging: wfx: simplify update of DTIM period

Current code parse the TIM and retrieve the DTIM period. It is far
easier to rely on bss_info_changed() for this job.

It is no more necessary to run task asynchronously. So
set_beacon_wakeup_period_work is now useless.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200115135338.14374-22-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>