MIPS: Use `FPU_CSR_ALL_X' in `__build_clear_fpe'

Replace a hardcoded numeric bitmask for FCSR cause bits with
`FPU_CSR_ALL_X' in `__build_clear_fpe'.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9684/


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