target/mips: Fix emulation of ST.W in system mode

Order of arguments in helper_ret_stl_mmu() invocations was wrong,
apparently caused by a misplaced multiline copy-and-paste.

Fixes: 6decc572 ("target/mips: Fix MSA instructions ST.<B|H|W|D> on big endian host")

Signed-off-by: Aleksandar Markovic <amarkovic@wavecomp.com>
Reviewed-by: Aleksandar Rikalo <arikalo@wavecomp.com>
Message-Id: <1567009239-11273-1-git-send-email-aleksandar.markovic@rt-rk.com>