MIPS: math-emu: Factor out NaN FP format conversions

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9686/


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