staging: rtl8723au: Remove RegBcnCtrlVal

The value saved in RegBcnCtrlVal was never used. In addition the code
was highly endian broken, declaring it as a u32, then accessing it via
a u8 * later.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>