MIPS: Remove invalid check

Unsigned values cannot be lesser than zero.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[ralf@linux-mips.org: Chris Dearman's original commit
9318c51a

 ([MIPS] MIPS32/MIPS64 secondary
cache management) introduced these less than zero checks in 2.6.18.]

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Cc: linux-kernel@vger.kernel.org
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Marek Szyprowski <m.szyprowski@samsung.com>
Cc: Markos Chandras <markos.chandras@imgtec.com>
Cc: Chris Dearman <chris.dearman@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11165/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>