risc-v: replace bitmap_weight with bitmap_empty in riscv_fill_hwcap()

bitmap_empty() is better than bitmap_weight() because it may return
earlier, and improves on readability.

CC: Albert Ou <aou@eecs.berkeley.edu>
CC: Anup Patel <anup@brainfault.org>
CC: Atish Patra <atishp@atishpatra.org>
CC: Jisheng Zhang <jszhang@kernel.org>
CC: Palmer Dabbelt <palmer@dabbelt.com>
CC: Paul Walmsley <paul.walmsley@sifive.com>
CC: Tsukasa OI <research_trasio@irq.a4lg.com>
CC: linux-riscv@lists.infradead.org
CC: linux-kernel@vger.kernel.org
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Reviewed-by: Anup Patel <anup@brainfault.org>