x86/IRQs: correct/tighten vector check in _clear_irq_vector()

x86/IRQs: correct/tighten vector check in _clear_irq_vector()

If any particular value was to be checked against, it would need to be
IRQ_VECTOR_UNASSIGNED.

Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Be more strict though and use valid_irq_vector() instead.

Take the opportunity and also convert local variables to unsigned int.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>