MIPS: Use CONFIG_CPU_MIPSR2 in csum_partial.S

The csum_partial implementation contain optimalizations for the MIPS R2
instruction set. This optimization is never enabled however because the
if directive uses the CPU_MIPSR2 constant which is not defined anywhere.

Use the CONFIG_CPU_MIPSR2 constant instead.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4971/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>