staging: wilc1000: use kernel provided struct cast to extract mac header

To parse the mac header make use of *struct ieee80211_hdr* instead of
extracting individual fields separately using pointer operation.

Signed-off-by: Ajay Singh <ajay.kathat@microchip.com>
Link: https://lore.kernel.org/r/20191122205153.30723-3-adham.abozaeid@microchip.com


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