staging: rtl8821ae: fix double const in sw.c

This commit fixes the following sparse warning:

drivers/staging/rtl8821ae/rtl8821ae/sw.c:
    - 449:14: warning: duplicate const

Signed-off-by: Konrad Zapalowicz <bergo.torino@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>