scsi-generic: Simplify error handling code

Coverity doesn't like the ignored return value introduced in
9d3b1551 (hw/block: Fix the return type), and other callers are
converted already in ceff3e1f.

This one was added lately in d9bcd6f7 and missed the train. Do it
now.

Signed-off-by: Fam Zheng <famz@redhat.com>
Message-Id: <20180118025245.13042-1-famz@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>