staging: wfx: drop wvif->dtim_period

It is not necessary to keep a copy of dtim_period in wfx_vif. Prefer to
just rely on bss_conf->dtim_period.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200115135338.14374-23-Jerome.Pouiller@silabs.com


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