MIPS: MT: Move an assignment for the variable "retval" in mipsmt_sys_sched_setaffinity()

MIPS: MT: Move an assignment for the variable "retval" in mipsmt_sys_sched_setaffinity()

A local variable was set to an error code in one case before a concrete
error situation was detected. Thus move the corresponding assignment into
an if branch to indicate a software failure there.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: linux-mips@linux-mips.org
Cc: LKML <linux-kernel@vger.kernel.org>
Cc: kernel-janitors@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/15072/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>