xtensa: disable IRQs while IRQ handler is running

IRQ handlers are expected to run with IRQs disabled.
See e.g. http://lwn.net/Articles/380931/

 for a longer story.

This was overlooked in the commit
  2d1c645c xtensa: dispatch medium-priority interrupts
Revert to old behavior and simplify interrupt entry and exit code.
Interrupt handler still honours IRQ priority.

do_notify_resume/schedule must be called with interrupts enabled, enable
interrupts if we return from user exception.

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