MIPS: lib: memset: Clean up some MIPS{EL,EB} ifdefery

The toolchain defines exactly one of __MIPSEB__ and
__MIPSEL__. As a result, simplify the ifdefery a little bit.

Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/8522/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>