MIPS: Fixup check for invalid scratch register

The invalid value for scratch register is -1, so update the checks of
the form (scratch_reg > 0) to be (scratch_reg >= 0).  This will fix
the case in Netlogic XLP where the scratch_reg can be 0.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/5444/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>