target-mips: Correct the handling of register #72 on writes

Fix an off-by-one error in `mips_cpu_gdb_write_register' for register
matching how `mips_cpu_gdb_read_register' handles it.  This register
slot is a fake anyway, there's nothing in hardware that corresponds to
it.

Signed-off-by: Maciej W. Rozycki <macro@codesourcery.com>
Reviewed-by: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
