xtensa: xtensa_intgen.py: Change 'not lvl in ...' to 'lvl not in ...'

Use the 'not in' operator. Fixes this pylint warning:

    arch/xtensa/core/xtensa_intgen.py:77:7: C0113: Consider changing
    "not lvl in ints_by_lvl" to "lvl not in ints_by_lvl" (unneeded-not)

Fixing pylint warnings for a CI check.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>