The pending register upper limit is currently set to
plic->num_sources >> 3, which is wrong, e.g.: considering
plic->num_sources is 7, the upper limit becomes 0 which fails
the range check if reading the pending register at pending_base.

Fixes: 1e24429e ("SiFive RISC-V PLIC Block")
Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20221211030829.802437-16-bmeng@tinylab.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>