MIPS: Replace hardcoded 32 with NUM_FPU_REGS in ptrace

NUM_FPU_REGS just makes it clearer what's going on, rather than the
magic hard coded 32.

Signed-off-by: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/6424/


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