minor improvements to Uio irq support functions
minor improvements to Uio irq support functions

- add generic irq_control method
- refactor irq_enable/irq_disable
- let irq_recv return the irq counter
- catch BlockingIOError from irq_recv, return None instead

mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio,mvduin/py-uio