target/mips: Style improvements in mips_mipssim.c

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

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1566216496-17375-17-git-send-email-aleksandar.markovic@rt-rk.com>