hw/intc/arm_gicv3_its: Convert int ID check to num_intids convention

hw/intc/arm_gicv3_its: Convert int ID check to num_intids convention

The bounds check on the number of interrupt IDs is correct, but
doesn't match our convention; change the variable name, initialize it
to the 2^n value rather than (2^n)-1, and use >= instead of > in the
comparison.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20220111171048.3545974-3-peter.maydell@linaro.org