Add a constant AHCI_MAX_IRQS removing magic number

Sponsored by:	Multiplay
Clamp ahci max irq's to AHCI_MAX_IRQS

This prevents the possiblity of any overruns on the statically allocated
struct irqs field.

Differential Revision:	D838
MFC after:	2 weeks
X-MFC-With:	r276012
Sponsored by:	Multiplay