Add a constant AHCI_MAX_IRQS removing magic number

Sponsored by:	Multiplay


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276012 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f
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


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276013 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f