xtensa: clean up do_interrupt/do_IRQ

- set up irq registers and call irq_enter/irq_exit once for each kernel
  entry due to interrupt;
- don't attempt to clear current IRQ in the do_interrupt, IRQ handler
  will take care of it;
- find pending interrupt with highest priority before every ISR
  invocation.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>