mips/boston: Report errors with error_report(), not error_printf()

Cc: Paul Burton <pburton@wavecomp.com>
Cc: Aleksandar Rikalo <arikalo@wavecomp.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190417190641.26814-5-armbru@redhat.com>