MIPS: math-emu: Declare function srl128() as static

MIPS: math-emu: Declare function srl128() as static

Declare function srl128() as static, since it it used just locally
to the source file.

This also removes a sparse warning for corresponding file.

Signed-off-by: Aleksandar Markovic <aleksandar.markovic@mips.com>
Cc: Douglas Leung <douglas.leung@mips.com>
Cc: Goran Ferenc <goran.ferenc@mips.com>
Cc: James Hogan <james.hogan@mips.com>
Cc: Miodrag Dinic <miodrag.dinic@mips.com>
Cc: Paul Burton <paul.burton@mips.com>
Cc: Petar Jovanovic <petar.jovanovic@mips.com>
Cc: Raghu Gandham <raghu.gandham@mips.com>
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/17585/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>