target/hppa: Use only low 2 immediate bits for PROBEI

During the conversion to decodetree, the 2-bit mask was lost.

Fixes: deee69a1 ("target/hppa: Convert memory management insns")
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>