Avoid unnecessary check for non-NULL Processor->Interrupt in EvtchnRelease()

Avoid unnecessary check for non-NULL Processor->Interrupt in EvtchnRelease()

If EvtchnIsProcessorEnabled() is TRUE then Processor->Interrupt should be
valid. Hence use an ASSERTion instead. Also replicate the check of
EvtchnIsProcessorEnabled() in the error path in EvtchnAcquire().
While we're at it, let's also use EvtchnIsProcessorEnabled() in
EvtchnInterruptEnable() and EvtchnInterruptDisable().

Signed-off-by: Paul Durrant <pdurrant@amazon.com>