staging: rtl8712: Fixed CamelCase wkFilterRxFF0 renamed to wk_filter_rx_ff0

In process of cleaning up struct _adapter in drv_types.h, wkFilterRxFF0
     is renamed to to wk_filter_rx_ff0 to fix a checkpatch reported issue.

Signed-off-by: Deepak Mishra <linux.dkm@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>