staging: rtl8192u: fix spaces in array indexing in r8192U_core.c

This patch fixes the following checkpatch error:
ERROR: space prohibited before open square bracket '['

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>