build: gen_isr_tables.py: fix bad IRQ index computation for L3 interrupts

According to the Zephyr documentation, the multi-level interrupts
are encoded as follows:

`L1_ID | ((L2_ID + 1) << L2_BITS) | ((L3_ID + 1) << (L3_BITS + L2_BITS))`

This means that when L3 interrupts are enabled, the gen_isr_table.py
script will receive the value of L2_ID + 1. Currently, the script
takes this value and directly compares it with the offsets set via
`CONFIG_3RD_LVL_INTR_xx_OFFSET`. This is wrong because the values from
said configurations are the same as L2_ID and because of that the
script will generate an error. To fix this, use the value of L2_ID
instead of L2_ID + 1.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>