target/nios2: Special case ipending in rdctl and wrctl

target/nios2: Special case ipending in rdctl and wrctl

It was never correct to be able to write to ipending.
Until the rest of the irq code is tidied, the read of
ipending will generate an "unnecessary" mask.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>