staging: rtl8712: Fixed CamelCase lockRxFF0Filter renamed to lock_rx_ff0_filter

In process of cleaning up struct _adapter in drv_types.h, lockRxFF0Filter
 is renamed to to lock_rx_ff0_filter to fix a checkpatch reported issue.

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