hw/misc: sifive_u_otp: Use error_report() when block operation fails

hw/misc: sifive_u_otp: Use error_report() when block operation fails

At present when blk_pread() / blk_pwrite() fails, a guest error
is logged, but this is not really a guest error. Change to use
error_report() instead.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 1611026585-29971-1-git-send-email-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>