riscv: Suppress the error report for QEMU testing with riscv_find_firmware()

riscv: Suppress the error report for QEMU testing with riscv_find_firmware()

We only ship plain binary bios images in the QEMU source. With Spike
machine that uses ELF images as the default bios, running QEMU test
will complain hence let's suppress the error report for QEMU testing.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Message-Id: <1588348254-7241-6-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>