drivers: intc: esp32: fix race in critical section locking

Replace global lock variable with function-local storage for irq_lock()
state. This fixes a race condition when multiple access enter or exit
the critical section at the same time. Now each lock/unlock pair uses
its own key, making interrupt safe.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>