hw/intc/arm_gicv3_its: Fix event ID bounds checks

hw/intc/arm_gicv3_its: Fix event ID bounds checks

In process_its_cmd() and process_mapti() we must check the
event ID against a limit defined by the size field in the DTE,
which specifies the number of ID bits minus one. Convert
this code to our num_foo convention:
 * change the variable names
 * use uint64_t and 1ULL when calculating the number
   of valid event IDs, because DTE.SIZE is 5 bits and
   so num_eventids may be up to 2^32
 * fix the off-by-one error in the comparison

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20220111171048.3545974-2-peter.maydell@linaro.org