staging: r8188eu: use ieee80211 helper to check for nullfunc frame

Use the ieee80211_is_nullfunc function from ieee80211.h to check for a
nullfunc frame. This is a data frame whose subtype has bit6 set.

Tested-by: Philipp Hortmann <philipp.g.hortmann@gmail.com> # Edimax N150
Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20220704201654.195574-7-martin@kaiser.cx


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