target/mips: Style improvements in machine.c

Fixes mostly errors and warnings reported by 'checkpatch.pl -f'.

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <1566216496-17375-10-git-send-email-aleksandar.markovic@rt-rk.com>