staging: rtl8723bs: core: fix line over 80 characters warning

Shorten the expression by re-using the part that was already computed to
fix the line over 80 characters warning reported by checkpatch.pl.

Signed-off-by: Anirudh Rayabharam <anirudh.rayabharam@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>