MIPS: math-emu: Optimise qNaN handling in `ieee754sp_fdp'

Rewrite qNaN handling in `ieee754sp_fdp' using the `ieee754_class_nan'
helper recently added, removing the external call to `ieee754sp_isnan'
and reducing the size of code by 16 instructions or 64 bytes.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9692/


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