riscv/sifive_u: Fix sifive_u_soc_realize() error API violations

riscv/sifive_u: Fix sifive_u_soc_realize() error API violations

The Error ** argument must be NULL, &error_abort, &error_fatal, or a
pointer to a variable containing NULL.  Passing an argument of the
latter kind twice without clearing it in between is wrong: if the
first call sets an error, it no longer points to NULL for the second
call.

sifive_u_soc_realize() is wrong that way: it passes &err to
sysbus_realize() four times before checking it.  Harmless, because the
first three can't actually fail (I think).

Fix by checking for failure right away.

Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: qemu-riscv@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20200630090351.1247703-18-armbru@redhat.com>